Overview
From Linux-raid
An Overview of Software-RAID
THIS AREA IS RAIDTOOLS ORIENTED - PLEASE EDIT TO CONVERT TO MDADM
This area of the wiki is based on "The Software RAID HowTo" by Jakob OEstergaard jakob@unthought.net and Emilio Bueso bueso@vives.org
This HOWTO describes how to use Software RAID under Linux. It addresses a specific version of the Software RAID layer, namely the 0.90 RAID layer currently maintained by Neil Brown. This is the RAID layer that is the standard in Linux-2.6 and Linux-2.4, and it is the version that is also used by Linux-2.2 kernels shipped from some vendors. The 0.90 RAID support is available as patches to Linux-2.0 and Linux-2.2, and is by many considered far more stable that the older RAID support already in those kernels.
Introduction
This Wiki focuses on the "new-style" RAID present in the 2.6 kernel series only. It does not describe the "old-style" RAID functionality present in 2.0 and 2.2 kernels although much of the functionality is available in the later 2.4 series kernels.
Disclaimer
The mandatory disclaimer:
All information herein is presented "as-is", with no warranties expressed nor implied. If you lose all your data, your job, get hit by a truck, whatever, it's not my fault, nor the developers'. Be aware, that you use the RAID software and this information at your own risk! There is no guarantee whatsoever, that any of the software, or this information, is in any way correct, nor suited for any use whatsoever. Back up all your data before experimenting with this. Better safe than sorry.
What is RAID?
In 1987, the University of California Berkeley, published an article entitled A Case for Redundant Arrays of Inexpensive Disks (RAID). This article described various types of disk arrays, referred to by the acronym RAID. The basic idea of RAID was to combine multiple small, independent disk drives into an array of disk drives which yields performance exceeding that of a Single Large Expensive Drive (SLED). Additionally, this array of drives appears to the computer as a single logical storage unit or drive.
The Mean Time Between Failure (MTBF) of the array will be equal to the MTBF of an individual drive, divided by the number of drives in the array. Because of this, the MTBF of an array of drives would be too low for many application requirements. However, disk arrays can be made fault-tolerant by redundantly storing information in various ways.
Five types of array architectures, RAID-1 through RAID-5, were defined by the Berkeley paper, each providing disk fault-tolerance and each offering different trade-offs in features and performance. In addition to these five redundant array architectures, it has become popular to refer to a non-redundant array of disk drives as a RAID-0 array.
Today some of the original RAID levels (namely level 2 and 3) are only used in very specialized systems (and in fact not even supported by the Linux Software RAID drivers). Another level, "linear" has emerged, and especially RAID level 0 is often combined with RAID level 1.
Terms
In this HOWTO the word "RAID" means "Linux Software RAID". This HOWTO does not treat any aspects of Hardware RAID. Furthermore, it does not treat any aspects of Software RAID in other operating system kernels.
When describing RAID setups, it is useful to refer to the number of disks and their sizes. At all times the letter N is used to denote the number of active disks in the array (not counting spare-disks). The letter S is the size of the smallest drive in the array, unless otherwise mentioned. The letter P is used as the performance of one disk in the array, in MB/s. When used, we assume that the disks are equally fast, which may not always be true in real-world scenarios.
Note that the words "device" and "disk" are supposed to mean about the same thing. Usually the devices that are used to build a RAID device are partitions on disks, not necessarily entire disks. But combining several partitions on one disk usually does not make sense, so the words devices and disks just mean "partitions on different disks".
The RAID levels
Here's a short description of what is supported in the Linux RAID drivers. Some of this information is absolutely basic RAID info, but I've added a few notices about what's special in the Linux implementation of the levels. You can safely skip this section if you know RAID already.
The current RAID drivers in Linux supports the following levels:
Linear mode
- Two or more disks are combined into one physical device. The disks are "appended" to each other, so writing linearly to the RAID device will fill up disk 0 first, then disk 1 and so on. The disks does not have to be of the same size. In fact, size doesn't matter at all here :)
- There is no redundancy in this level. If one disk crashes you will most probably lose all your data. You can however be lucky to recover some data, since the filesystem will just be missing one large consecutive chunk of data.
- The read and write performance will not increase for single reads/writes. But if several users use the device, you may be lucky that one user effectively is using the first disk, and the other user is accessing files which happen to reside on the second disk. If that happens, you will see a performance gain.
RAID-0
- Also called "stripe" mode. The devices should (but need not) have the same size. Operations on the array will be split on the devices; for example, a large write could be split up as 64 kiB to disk 0, 64 kiB to disk 1, 64 kiB to disk 2, then 64 kiB to disk 0 again, and so on. Writes to each disk will go on at the same time. If one device is much larger than the other devices, that extra space is still utilized in the RAID device, but you will be accessing this larger disk alone, during writes in the high end of your RAID device. This of course hurts performance.
- Like linear, there is no redundancy in this level either. Unlike linear mode, you will not be able to rescue any data if a drive fails. If you remove a drive from a RAID-0 set, the RAID device will not just miss one consecutive block of data, it will be filled with small holes all over the device. e2fsck or other filesystem recovery tools will probably not be able to recover much from such a device.
- The read and write performance will increase, because reads and writes are done in parallel on the devices. This is usually the main reason for running RAID-0. If the busses to the disks are fast enough, you can get very close to N*P MB/sec.
RAID-1
- This is the first mode which actually has redundancy. RAID-1 can be used on two or more disks with zero or more spare-disks. This mode maintains an exact mirror of the information on one disk on the other disk(s). Of Course, the disks must be of equal size. If one disk is larger than another, your RAID device will be the size of the smallest disk.
- If up to N-1 disks are removed (or crashes), all data are still intact. If there are spare disks available, and if the system (eg. SCSI drivers or IDE chipset etc.) survived the crash, reconstruction of the mirror will immediately begin on one of the spare disks, after detection of the drive fault.
- Write performance is often worse than on a single device, because identical copies of the data written must be sent to every disk in the array. With large RAID-1 arrays this can be a real problem, as you may saturate the PCI bus with these extra copies. This is in fact one of the very few places where Hardware RAID solutions can have an edge over Software solutions - if you use a hardware RAID card, the extra write copies of the data will not have to go over the PCI bus, since it is the RAID controller that will generate the extra copy. Read performance is good, especially if you have multiple readers or seek- intensive workloads. The RAID code employs a rather good read- balancing algorithm, that will simply let the disk whose heads are closest to the wanted disk position perform the read operation. Since seek operations are relatively expensive on modern disks (a seek time of 8 ms equals a read of 640 kB at 80 MB/sec), picking the disk that will have the shortest seek time does actually give a noticeable performance improvement.
RAID-4
- This RAID level is not used very often. It can be used on three or more disks. Instead of completely mirroring the information, it keeps parity information on one drive, and writes data to the other disks in a RAID-0 like way. Because one disk is reserved for parity information, the size of the array will be (N-1)*S, where S is the size of the smallest drive in the array. As in RAID-1, the disks should either be of equal size, or you will just have to accept that the S in the (N-1)*S formula above will be the size of the smallest drive in the array.
- If one drive fails, the parity information can be used to reconstruct all data. If two drives fail, all data is lost.
- The reason this level is not more frequently used, is because the parity information is kept on one drive. This information must be updated every time one of the other disks are written to. Thus, the parity disk will become a bottleneck, if it is not a lot faster than the other disks. However, if you just happen to have a lot of slow disks and a very fast one, this RAID level can be very useful.
RAID-5
- This is perhaps the most useful RAID mode when one wishes to combine a larger number of physical disks, and still maintain some redundancy. RAID-5 can be (usefully) used on three or more disks, with zero or more spare-disks. The resulting RAID-5 device size will be (N-1)*S, just like RAID-4. The big difference between RAID-5 and -4 is, that the parity information is distributed evenly among the participating drives, avoiding the bottleneck problem in RAID-4, and also getting more performance out of the disk when reading, as all drives will then be used.
- If one of the disks fail, all data are still intact, thanks to the parity information. If spare disks are available, reconstruction will begin immediately after the device failure. If two disks fail simultaneously, or before the raid is reconstructed, all data are lost. RAID-5 can survive one disk failure, but not two or more.
- Both read and write performance usually increase, but can be hard to predict how much. Reads are almost similar to RAID-0 reads, writes can be either rather expensive (requiring read-in prior to write, in order to be able to calculate the correct parity information, such as in database operations), or similar to RAID-1 writes (when larger sequential writes are performed, and parity can be calculated directly from the other blocks to be written). The write efficiency depends heavily on the amount of memory in the machine, and the usage pattern of the array. Heavily scattered writes are bound to be more expensive.
RAID-6
- This is an extension of RAID-5 to provide more resilience. RAID-6 can be (usefully) used on four or more disks, with zero or more spare-disks. The resulting RAID-6 device size will be (N-2)*S. The big difference between RAID-5 and -6 is that there are two different parity information blocka, and these are distributed evenly among the participating drives.
- Since there are two parity blocks; if one or two of the disks fail, all data is still intact. If spare disks are available, reconstruction will begin immediately after the device failure(s).
- Read performance is almost similar to RAID-5 but write performance is worse.
RAID-10
- RAID-10 is an 'in-kernel' combination of RAID-1 and RAID-0 that is more efficient than simply layering RAID levels.
- RAID-10 has a layout ('far') which can provide sequential read throughput that scales by number of drives, rather than number of RAID-1 pairs. You can get about 95 % of the performance og the RAID-0 with same amount of drives.
- RAID-10 allows spare disk(s) to be shared amongst all the raid1 pairs.
FAULTY
- This is a special debugging RAID level. It only allows one device and simulates low level read/write failures.
- Using a FAULTY device in another RAID level allows administrators to practice dealing with things like sector-failures as opposed to whole drive failures
Requirements
This HOWTO assumes you are using Linux 2.6 or later and the latest toolset.
If you use and recent GNU/Linux distribution based on the 2.4 kernel or later, your system most likely already has a matching version of mdadm for your kernel.
Note that according to it's homepage: http://people.redhat.com/mingo/raidtools/ raidtools hasn't been updated since Jan 2003 and is deprecated in favour of mdadm.
Why RAID?
There can be many good reasons for using RAID. A few are; the ability to combine several physical disks into one larger "virtual" device, performance improvements, and redundancy.
It is, however, very important to understand that RAID is not a general substitute for good backups. Some RAID levels will make your systems immune to data loss from one or two disk failures, but RAID will not allow you to recover from an accidental "rm -rf /". RAID will also not help you preserve your data if the server holding the RAID itself is lost in one way or the other (theft, flooding, earthquake, Martian invasion etc.)
RAID will generally allow you to keep systems up and running, in case of common hardware problems (disk failure). It is not in itself a complete data safety solution. This is very important to realize.
Device and filesystem support
Linux RAID can work on most block devices. It doesn't matter whether you use SATA, USB, IDE or SCSI devices, or a mixture. Some people have also used the Network Block Device (NBD) with success.
Since a Linux Software RAID device is itself a block device, the above implies that you can actually create a RAID of other RAID devices. This in turn makes it possible to support RAID-10 (RAID-0 of multiple RAID-1 devices), simply by using the RAID-0 and RAID-1 functionality together. Other more exotic configurations, such a RAID-5 over RAID-5 "matrix" configurations are equally supported.
The RAID layer has absolutely nothing to do with the filesystem layer. You can put any filesystem on a RAID device, just like any other block device.
Performance
Often RAID is employed as a solution to performance problems. While RAID can indeed often be the solution you are looking for, it is not a silver bullet. There can be many reasons for performance problems, and RAID is only the solution to a few of them.
See the Introduction#The_RAID_levels for a mention of the performance characteristics of each level.
Swapping on RAID
Swapping on a mirrored RAID can help you survive a failing disk. If a disk fails, then data for swapped processes would be inaccessable in a non-mirrored environment. If you run in a mirrored environment, then the system can go on running even if a disk fails in service. You can even have more than one copy of the data with a raid10 type array, preventing against multiple disks failing.
There's not much reason to use RAID0 for swap performance reasons. The kernel itself can stripe swapping on several devices, if you just give them the same priority in the /etc/fstab file.
A nice /etc/fstab could look like:
/dev/sda2 swap swap defaults,pri=1 0 0 /dev/sdb2 swap swap defaults,pri=1 0 0 /dev/sdc2 swap swap defaults,pri=1 0 0 /dev/sdd2 swap swap defaults,pri=1 0 0 /dev/sde2 swap swap defaults,pri=1 0 0 /dev/sdf2 swap swap defaults,pri=1 0 0 /dev/sdg2 swap swap defaults,pri=1 0 0
This setup lets the machine swap in parallel on seven SCSI devices. No
need for RAID0, since this has been a kernel feature for a long time.
A different reason to use RAID for swap is high availability. If you set up a system to boot on eg. a RAID-1 device, the system should be able to survive a disk crash. If a system without mirrored swapping has been swapping on the now faulty device, you will most likely be going down. Swapping on a mirrored RAID partition such as RAID-1, raid10,n2 or raid10,f2 type would solve this problem.
There has been a lot of discussion about whether swap was stable on RAID devices. This is a continuing debate, because it depends highly on other aspects of the kernel as well. As of this writing, it seems that swapping on RAID should be perfectly stable, you should however stress-test the system yourself until you are satisfied with the stability.
You can set up RAID in a swap file on a filesystem on your RAID device, or you can set up a RAID device as a swap partition, as you see fit. As usual, the RAID device is just a block device.
Why mdadm?
mdadm is now the standard software RAID management tool for Linux; using raidtools is deprecated (and the code does not appear to have been worked on since Jan 2003) and although it will still work for basic use, bugs have been reported where raidtools does not handle new features correctly.
The mdadm tool, written by Neil Brown, who is a software engineer at the University of New South Wales and a kernel developer. See http://www.kernel.org/pub/linux/utils/raid/mdadm/ANNOUNCE for the latest version.
The main differences between mdadm and raidtools are:
- mdadm can diagnose, monitor and gather detailed information about your arrays
- mdadm is a single centralized program and not a collection of disperse programs, so there's a common syntax for every RAID management command
- mdadm can perform almost all of its functions without having a configuration file and does not use one by default
- Also, if a configuration file is needed, mdadm will help with management of it's contents
Devices
Software RAID devices are so-called "block" devices, like ordinary disks or disk partitions. A RAID device is "built" from a number of other block devices - for example, a RAID-1 could be built from two ordinary disks, or from two disk partitions (on separate disks - please see the description of RAID-1 for details on this).
There are no other special requirements to the devices from which you build your RAID devices - this gives you a lot of freedom in designing your RAID solution. For example, you can build a RAID from a mix of IDE and SCSI devices, and you can even build a RAID from other RAID devices (this is useful for RAID-0+1, where you simply construct two RAID-1 devices from ordinary disks, and finally construct a RAID-0 device from those two RAID-1 devices).
Therefore, in the following text, we will use the word "device" as meaning "disk", "partition", or even "RAID device". A "device" in the following text simply refers to a "Linux block device". It could be anything from a SCSI disk to a network block device. We will commonly refer to these "devices" simply as "disks", because that is what they will be in the common case.
However, there are several roles that devices can play in your arrays. A device could be a "spare disk", it could have failed and thus be a "faulty disk", or it could be a normally working and fully functional device actively used by the array.
In the following we describe two special types of devices; namely the "spare disks" and the "faulty disks".
It is worth mentioning the existence of the FAULTY RAID level - don't get confused - this is a special debugging level of RAID that uses a normal device and simulates faults.
Spare disks
Spare disks (often called hot spares) are disks that do not take part in the RAID set until one of the active disks fail. When a device failure is detected, that device is marked as "faulty" and reconstruction is immediately started on the first spare disk available.
Thus, spare disks add a nice extra safety to especially RAID-5 systems that perhaps are hard to get to (physically). One can allow the system to run for some time, with a faulty device, since the spare disk takes the place of the faulty device and all redundancy is restored.
It is also possible to have spare disks spin-down to save energy; obviously the spin-up time for these warm spares is insignificant compared to the resync time.
You cannot be sure that your system will keep running after a disk crash though. The RAID layer should handle device failures just fine, but SCSI drivers could be broken on error handling, or the IDE chipset could lock up, or a lot of other things could happen.
Also, once reconstruction to a hot-spare begins, the RAID layer will start reading from all the other disks to re-create the redundant information. If multiple disks have built up bad blocks over time, the reconstruction itself can actually trigger a failure on one of the "good" disks. This can lead to a complete RAID failure and is the major reason for using RAID-6 in preference to RAID-5 and a hot spare.
If you do frequent backups of the entire filesystem on the RAID array, then it is highly unlikely that you would ever get in this situation - this is another very good reason for taking frequent backups. Remember, RAID is not a substitute for backups.
Faulty disks
When the RAID layer handles device failures just fine, crashed disks are marked as faulty, and reconstruction is immediately started on the first spare-disk available. If no spare is available then the array runs in 'degraded' mode.
Faulty disks still appear and behave as members of the array. The RAID layer just avoids reading/writing them.
If a device needs to be removed from an array for any reason (eg pro-active replacement due to SMART reports) then it must be marked as faulty before it can be removed.
The section on Detecting, querying and testing provides more information.
Hardware issues
This section will mention some of the hardware concerns involved when running software RAID.
If you are going after high performance, you should make sure that the bus(ses) to the drives are fast enough. You should not have 14 UW-SCSI drives on one UW bus, if each drive can give 20 MB/s and the bus can only sustain 160 MB/s. Also, you should only have one device per IDE channel. Running disks as master/slave is horrible for performance. IDE is really bad at accessing more that one drive per channel. Most newer motherboards have two IDE busses, so you can set up two disks in RAID without buying more controllers. Extra IDE controllers are rather cheap these days, so setting up 6-8 disk systems with IDE is easy and affordable.
Also most newer motherboards have one or more controllers for SATA disks. SATA disks do not connect more than one disk on one channel so the problems mentined aboce with multiple IO for 2 disks on one channel is not relevant for SATA disks.
See also the section on bottlenecks.
IDE Configuration
It is indeed possible to run RAID over IDE disks. And excellent performance can be achieved too. In fact, today's price on IDE drives and controllers does make IDE something to be considered, when setting up new RAID systems.
- Physical stability: IDE drives has traditionally been of lower mechanical quality than SCSI drives. Even today, the warranty on IDE drives is typically one year, whereas it is often three to five years on SCSI drives. Although it is not fair to say, that IDE drives are per definition poorly made, one should be aware that IDE drives of some brand may fail more often that similar SCSI drives. However, other brands use the exact same mechanical setup for both SCSI and IDE drives. It all boils down to: All disks fail, sooner or later, and one should be prepared for that.
- Data integrity: Earlier, IDE had no way of assuring that the data sent onto the IDE bus would be the same as the data actually written to the disk. This was due to total lack of parity, checksums, etc. With the Ultra-DMA standard, IDE drives now do a checksum on the data they receive, and thus it becomes highly unlikely that data get corrupted. The PCI bus however, does not have parity or checksum, and that bus is used for both IDE and SCSI systems.
- Performance: I am not going to write thoroughly about IDE performance here. The really short story is:
- IDE drives are fast, although they are not (as of this writing) found in 10.000 or 15.000 rpm versions as their SCSI counterparts
- IDE has more CPU overhead than SCSI (but who cares?)
- Only use one IDE drive per IDE bus, slave disks spoil performance
- Fault survival: The IDE driver usually survives a failing IDE device. The RAID layer will mark the disk as failed, and if you are running RAID levels 1 or above, the machine should work just fine until you can take it down for maintenance.
It is very important, that you only use one IDE disk per IDE bus. Not only would two disks ruin the performance, but the failure of a disk often guarantees the failure of the bus, and therefore the failure of all disks on that bus. In a fault-tolerant RAID setup (RAID levels 1,4,5), the failure of one disk can be handled, but the failure of two disks (the two disks on the bus that fails due to the failure of the one disk) will render the array unusable. Also, when the master drive on a bus fails, the slave or the IDE controller may get awfully confused. One bus, one drive, that's the rule.
There are cheap PCI IDE controllers out there. You often get two or four busses for around $80. Considering the much lower price of IDE disks versus SCSI disks, an IDE disk array can often be a really nice solution if one can live with the relatively low number (around 8 probably) of disks one can attach to a typical system.
IDE has major cabling problems when it comes to large arrays. Even if you had enough PCI slots, it's unlikely that you could fit much more than 8 disks in a system and still get it running without data corruption caused by too long IDE cables.
Furthermore, some of the newer IDE drives come with a restriction that they are only to be used a given number of hours per day. These drives are meant for desktop usage, and it can lead to severe problems if these are used in a 24/7 server RAID environment.
Hot Swap
Although hot swapping of drives is supported to some extent, it is still not something one can do easily.
Hot-swapping IDE drives
Don't ! IDE doesn't handle hot swapping at all. Sure, it may work for you, if your IDE driver is compiled as a module (only possible in the 2.2 series of the kernel), and you re-load it after you've replaced the drive. But you may just as well end up with a fried IDE controller, and you'll be looking at a lot more down-time than just the time it would have taken to replace the drive on a downed system.
The main problem, except for the electrical issues that can destroy your hardware, is that the IDE bus must be re-scanned after disks are swapped. While newer Linux kernels do support re-scan of an IDE bus (with the help of the hdparm utility), re-detecting partitions is still something that is lacking. If the new disk is 100% identical to the old one (wrt. geometry etc.), it may work, but really, you are walking the bleeding edge here.
Hot-swapping SCSI drives
Normal SCSI hardware is not hot-swappable either. It may however work. If your SCSI driver supports re-scanning the bus, and removing and appending devices, you may be able to hot-swap devices. However, on a normal SCSI bus you probably shouldn't unplug devices while your system is still powered up. But then again, it may just work (and you may end up with fried hardware).
The SCSI layer should survive if a disk dies, but not all SCSI drivers handle this yet. If your SCSI driver dies when a disk goes down, your system will go with it, and hot-plug isn't really interesting then.
Hot-swapping with SATA
SATA supports hot swapping a drive but the linux kernel is not quite there yet.
see http://linux-ata.org/driver-status.html for more information on SATA hotpug status.
Hot-swapping with SCA
With SCA, it is possible to hot-plug devices. Unfortunately, this is not as simple as it should be, but it is both possible and safe.
Replace the RAID device, disk device, and host/channel/id/lun numbers with the appropriate values in the example below:
- Dump the partition table from the drive, if it is still readable:
sfdisk -d /dev/sdb > partitions.sdb
- Mark faulty and remove the drive to replace from the array:
mdadm -f /dev/md0 /dev/sdb1
mdadm -r /dev/md0 /dev/sdb1
- Look up the Host, Channel, ID and Lun of the drive to replace, by looking in
/proc/scsi/scsi
- Remove the drive from the bus:
echo "scsi remove-single-device 0 0 2 0" > /proc/scsi/scsi
- Verify that the drive has been correctly removed, by looking in
/proc/scsi/scsi
- Unplug the drive from your SCA bay, and insert a new drive
- Add the new drive to the bus:
echo "scsi add-single-device 0 0 2 0" > /proc/scsi/scsi
(this should spin up the drive as well)
- Re-partition the drive using the previously dumped partition table:
sfdisk /dev/sdb < partitions.sdb
- Add the drive to your array:
mdadm -a /dev/md0 /dev/sdb1
The arguments to the "scsi remove-single-device" commands are: Host, Channel, Id and Lun. These numbers are found in the "/proc/scsi/scsi" file.
The above steps have been tried and tested on a system with IBM SCA disks and an Adaptec SCSI controller. If you encounter problems or find easier ways to do this, please discuss this on the linux-raid mailing list.
RAID setup
General setup
This is what you need for any of the RAID levels:
- A kernel with the appropriate md support either as modules or built-in. Preferably a kernel from the 2.6 series. Alternatively a stable 2.4 kernel (pre 2.4 kernels are no longer covered in this document).
- The mdadm tool
- Patience, Pizza, and your favorite caffeinated beverage.
All of this is included as standard in most GNU/Linux distributions today.
If your system has RAID support, you should have a file called /proc/mdstat. Remember it, that file is your friend. If you do not have that file, maybe your kernel does not have RAID support. See what the contains, by doing a
cat /proc/mdstat
It should tell you that you have the right RAID personality (eg. RAID mode) registered, and that no RAID devices are currently active. See the /proc/mdstat page for more details.
Preparing and partitioning your disk devices
Arrays can be built on top of entire disks or on partitions.
This leads to 2 frequent questions:
- Should I use entire device or a partition?
- What partition type?
Which are discussed in Partition Types
Downloading and installing mdadm - the RAID management tool
mdadm is now the standard RAID management tool and should be found in any modern distribution.
You can download the most recent mdadm tarball at http://www.cse.unsw.edu.au/~neilb/source/mdadm/.
Use the normal distribution method for obtaining the package:
Debian:
apt-get install mdadm
Gentoo:
emerge mdadm
Understanding mdadm
mdadm is well documented in its manpage - well worth a read.
mdadm has 7 major modes of operation:
Create
Create a new array with per-device superblocks (normal creation).
Assemble
Assemble the parts of a previously created array into an active array. Components can be explicitly given or can be searched for. mdadm checks that the components do form a bona fide array, and can, on request, fiddle superblock information so as to assemble a faulty array. Typically you do this in the init scripts after rebooting.
Follow or Monitor
Monitor one or more md devices and act on any state changes. This is only meaningful for raid1, 4, 5, 6, 10 or multipath arrays as only these have interesting state. raid0 or linear never have missing, spare, or failed drives, so there is nothing to monitor. Typically you do this after rebooting too.
Build
Build an array that doesn't have per-device superblocks. For these sorts of arrays, mdadm cannot differentiate between initial creation and subsequent assembly of an array. It also cannot perform any checks that appropriate devices have been requested. Because of this, the Build mode should only be used together with a complete understanding of what you are doing.
Grow
Grow (or shrink) an array, or otherwise reshape it in some way. Currently supported growth options including changing the active size of component devices in RAID level 1/4/5/6 and changing the number of active devices in RAID1.
Manage
This is for doing things to specific components of an array such as adding new spares and removing faulty devices.
Misc
This is an 'everything else' mode that supports operations on active arrays, operations on component devices such as erasing old superblocks, and information gathering operations.
Normal operation just uses the 'Create', 'Assemble' and 'Monitor' commands - the rest come in handy when you're messing with your array; typically fixing it or changing it.
Below we'll see how to create arrays of various types; the basic approach is:
mdadm --create /dev/md0 <blah> mdadm --monitor /dev/md0
Linear mode
Ok, so you have two or more partitions which are not necessarily the same size (but of course can be), which you want to append to each other.
Spare-disks are not supported here. If a disk dies, the array dies with it. There's no information to put on a spare disk.
You're probably wondering why we specify a chunk-size here when linear mode just appends the disks into one large array with no parallelism. Well, you're completely right, it's odd. Just put in some chunk size and don't worry about this any more.
Using mdadm, a single command like
mdadm --create --verbose /dev/md0 --level=linear --raid-devices=2 /dev/sdb6 /dev/sdc5
should create the array. The parameters talk for themselves. The out- put might look like this
mdadm: chunk size defaults to 64K mdadm: array /dev/md0 started.
Have a look in /proc/mdstat. You should see that the array is running.
Now, you can create a filesystem, just like you would on any other device, mount it, include it in your /etc/fstab and so on.
RAID-0
You have two or more devices, of approximately the same size, and you want to combine their storage capacity and also combine their performance by accessing them in parallel.
mdadm --create --verbose /dev/md0 --level=stripe --raid-devices=2 /dev/sdb6 /dev/sdc5
Like in Linear mode, spare disks are not supported here either. RAID-0 has no redundancy, so when a disk dies, the array goes with it.
Having run mdadm you have initialised the superblocks and started the raid device. Have a look in /proc/mdstat to see what's going on. You should see that your device is now running.
/dev/md0 is now ready to be formatted, mounted, used and abused.
RAID-1
You have two devices of approximately same size, and you want the two to be mirrors of each other. Eventually you have more devices, which you want to keep as stand-by spare-disks, that will automatically become a part of the mirror if one of the active devices break.
mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1
If you have spare disks, you can add them to the end of the device specification like
mdadm --create --verbose /dev/md0 --level=mirror --raid-devices=2 /dev/sdb1 /dev/sdc1 --spare-devices=1 /dev/sdd1
Ok, now we're all set to start initializing the RAID. The mirror must be constructed, eg. the contents (however unimportant now, since the device is still not formatted) of the two devices must be synchronized.
Check out the /proc/mdstat file. It should tell you that the /dev/md0 device has been started, that the mirror is being reconstructed, and an ETA of the completion of the reconstruction.
Reconstruction is done using idle I/O bandwidth. So, your system should still be fairly responsive, although your disk LEDs should be glowing nicely.
The reconstruction process is transparent, so you can actually use the device even though the mirror is currently under reconstruction.
Try formatting the device, while the reconstruction is running. It will work. Also you can mount it and use it while reconstruction is running. Of Course, if the wrong disk breaks while the reconstruction is running, you're out of luck.
RAID-5
You have three or more devices of roughly the same size, you want to combine them into a larger device, but still to maintain a degree of redundancy for data safety. Eventually you have a number of devices to use as spare-disks, that will not take part in the array before another device fails.
If you use N devices where the smallest has size S, the size of the entire array will be (N-1)*S. This "missing" space is used for parity (redundancy) information. Thus, if any disk fails, all data stay intact. But if two disks fail, all data is lost.
A chunk size of 32 kB is a good default for many general purpose filesystems of this size. The array on which the above raidtab is used, is a 7 times 6 GB = 36 GB (remember the (n-1)*s = (7-1)*6 = 36) device. It holds an ext2 filesystem with a 4 kB block size. You could go higher with both array chunk-size and filesystem block-size if your filesystem is either much larger, or just holds very large files.
Creating an array
Ok, enough talking. Let's see if it works. Run your command:
mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/sdb1 /dev/sdc1 /dev/sdd1 --spare-devices=1 /dev/sde1
and see what happens. Hopefully your disks start working like mad, as they begin the reconstruction of your array. Have a look in /proc/mdstat to see what's going on.
If the device was successfully created, the reconstruction process has now begun. Your array is not consistent until this reconstruction phase has completed. However, the array is fully functional (except for the handling of device failures of course), and you can format it and use it even while it is reconstructing.
See the section on special options for mke2fs before formatting the array.
Using the Array
At this point you should be able to create a simple array of any flavour (hint: --level is your friend)
Ok, now when you have your RAID device running, you can always stop it:
mdadm --stop /dev/md0
Starting is a little more complex; you may think that:
mdadm --run /dev/md0
would work - but it doesn't.
Linux raid devices don't really exist on their own; they have to be assembled each time you want to use them. Assembly is like creation insofar as it pulls together devices
If you earlier ran:
mdadm --create /dev/md0 --level=5 --raid-devices=4 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
then
mdadm --assemble /dev/md0 /dev/sda1 /dev/sdb1 /dev/sdc1 /dev/sdd1
would work.
However, the easy way to do this if you have a nice simple setup is:
mdadm --scan --assemble
For complex cases (ie you pull in disks from other machines that you're trying to repair) this has the potential to start arrays you don't really want started. A safer mechanism is to use the uuid parameter and run:
mdadm --scan --assemble --uuid=a26bf396:31389f83:0df1722d:f404fe4c
This will only assemble the array that you want - but it will work no matter what has happened to the device names. This is particularly cool if, for example, you add in a new SATA controller card and all of a sudden /dev/sda becomes /dec/sde!!!
The Persistent Superblock
Back in "The Good Old Days" (TM), the raidtools would read your /etc/raidtab file, and then initialize the array. However, this would require that the filesystem on which /etc/raidtab resided was mounted. This was unfortunate if you want to boot on a RAID.
Also, the old approach led to complications when mounting filesystems on RAID devices. They could not be put in the /etc/fstab file as usual, but would have to be mounted from the init-scripts.
The persistent superblocks solve these problems. When an array is created with the persistent-superblock option (the default now), a special superblock is written to a location (different for different superblock versions) on all disks participating in the array. This allows the kernel to read the configuration of RAID devices directly from the disks involved, instead of reading from some configuration file that may not be available at all times.
It's not a bad idea to maintain a consistent /etc/mdadm.conf file, since you may need this file for later recovery of the array.
The persistent superblock is mandatory if you want auto-detection of your RAID devices upon system boot. This is described in the Autodetect section.
Superblock physical layouts are listed on RAID superblock formats .
Chunk sizes
The chunk-size deserves an explanation. You can never write completely parallel to a set of disks. If you had two disks and wanted to write a byte, you would have to write four bits on each disk. Actually, every second bit would go to disk 0 and the others to disk 1. Hardware just doesn't support that. Instead, we choose some chunk- size, which we define as the smallest "atomic" mass of data that can be written to the devices. A write of 16 kB with a chunk size of 4 kB will cause the first and the third 4 kB chunks to be written to the first disk and the second and fourth chunks to be written to the second disk, in the RAID-0 case with two disks. Thus, for large writes, you may see lower overhead by having fairly large chunks, whereas arrays that are primarily holding small files may benefit more from a smaller chunk size.
Chunk sizes must be specified for all RAID levels, including linear mode. However, the chunk-size does not make any difference for linear mode.
For optimal performance, you should experiment with the chunk-size, as well as with the block-size of the filesystem you put on the array.
The argument to the chunk-size option in /etc/raidtab specifies the chunk-size in kilobytes. So "4" means "4 kB".
RAID-0
Data is written "almost" in parallel to the disks in the array. Actually, chunk-size bytes are written to each disk, serially.
If you specify a 4 kB chunk size, and write 16 kB to an array of three disks, the RAID system will write 4 kB to disks 0, 1 and 2, in parallel, then the remaining 4 kB to disk 0.
A 32 kB chunk-size is a reasonable starting point for most arrays. But the optimal value depends very much on the number of drives involved, the content of the file system you put on it, and many other factors. Experiment with it, to get the best performance.
RAID-0 with ext2
The following tip was contributed by michael@freenet-ag.de:
There is more disk activity at the beginning of ext2fs block groups. On a single disk, that does not matter, but it can hurt RAID0, if all block groups happen to begin on the same disk. Example:
With 4k stripe size and 4k block size, each block occupies one stripe. With two disks, the stripe-#disk-product is 2*4k=8k. The default block group size is 32768 blocks, so all block groups start on disk 0, which can easily become a hot spot, thus reducing overall performance. Unfortunately, the block group size can only be set in steps of 8 blocks (32k when using 4k blocks), so you can not avoid the problem by adjusting the block group size with the -g option of mkfs(8).
If you add a disk, the stripe-#disk-product is 12, so the first block group starts on disk 0, the second block group starts on disk 2 and the third on disk 1. The load caused by disk activity at the block group beginnings spreads over all disks.
In case you can not add a disk, try a stripe size of 32k. The stripe-#disk-product is 64k. Since you can change the block group size in steps of 8 blocks (32k), using a block group size of 32760 solves the problem.
Additionally, the block group boundaries should fall on stripe boundaries. That is no problem in the examples above, but it could easily happen with larger stripe sizes.
RAID-1
For writes, the chunk-size doesn't affect the array, since all data must be written to all disks no matter what. For reads however, the chunk-size specifies how much data to read serially from the participating disks. Since all active disks in the array contain the same information, the RAID layer has complete freedom in choosing from which disk information is read - this is used by the RAID code to improve average seek times by picking the disk best suited for any given read operation.
RAID-4
When a write is done on a RAID-4 array, the parity information must be updated on the parity disk as well.
The chunk-size affects read performance in the same way as in RAID-0, since reads from RAID-4 are done in the same way.
RAID-5
On RAID-5, the chunk size has the same meaning for reads as for RAID-0. Writing on RAID-5 is a little more complicated: When a chunk is written on a RAID-5 array, the corresponding parity chunk must be updated as well. Updating a parity chunk requires either
- The original chunk, the new chunk, and the old parity block
- Or, all chunks (except for the parity chunk) in the stripe
The RAID code will pick the easiest way to update each parity chunk as the write progresses. Naturally, if your server has lots of memory and/or if the writes are nice and linear, updating the parity chunks will only impose the overhead of one extra write going over the bus (just like RAID-1). The parity calculation itself is extremely efficient, so while it does of course load the main CPU of the system, this impact is negligible. If the writes are small and scattered all over the array, the RAID layer will almost always need to read in all the untouched chunks from each stripe that is written to, in order to calculate the parity chunk. This will impose extra bus-overhead and latency due to extra reads.
A reasonable chunk-size for RAID-5 is 128 kB, but as always, you may want to experiment with this.
Also see the section on special options for mke2fs. This affects RAID-5 performance.
Options for mke2fs
There is a special option available when formatting RAID-4 or -5 devices with mke2fs. The -R stride=nn option will allow mke2fs to better place different ext2 specific data-structures in an intelligent way on the RAID device.
If the chunk-size is 32 kB, it means, that 32 kB of consecutive data will reside on one disk. If we want to build an ext2 filesystem with 4 kB block-size, we realize that there will be eight filesystem blocks in one array chunk. We can pass this information on the mke2fs utility, when creating the filesystem:
mke2fs -b 4096 -R stride=8 /dev/md0
RAID-{4,5,10} performance is severely influenced by this option. I am
unsure how the stride option will affect other RAID levels. If anyone
has information on this, please send it in my direction.
The ext2fs blocksize severely influences the performance of the filesystem. You should always use 4kB block size on any filesystem larger than a few hundred megabytes, unless you store a very large number of very small files on it.
Detecting, querying and testing
This section is about life with a software RAID system, that's communicating with the arrays and tinkertoying them.
Note that when it comes to md devices manipulation, you should always remember that you are working with entire filesystems. So, although there could be some redundancy to keep your files alive, you must proceed with caution.
Detecting a drive failure
Firstly: mdadm has an excellent 'monitor' mode which will send an email when a problem is detected in any array (more about that later).
Of course the standard log and stat files will record more details about a drive failure.
It's always a must for /var/log/messages to fill screens with tons of error messages, no matter what happened. But, when it's about a disk crash, huge lots of kernel errors are reported. Some nasty examples, for the masochists,
kernel: scsi0 channel 0 : resetting for second half of retries.
kernel: SCSI bus is being reset for host 0 channel 0.
kernel: scsi0: Sending Bus Device Reset CCB #2666 to Target 0
kernel: scsi0: Bus Device Reset CCB #2666 to Target 0 Completed
kernel: scsi : aborting command due to timeout : pid 2649, scsi0, channel 0, id 0, lun 0 Write (6) 18 33 11 24 00
kernel: scsi0: Aborting CCB #2669 to Target 0
kernel: SCSI host 0 channel 0 reset (pid 2644) timed out - trying harder
kernel: SCSI bus is being reset for host 0 channel 0.
kernel: scsi0: CCB #2669 to Target 0 Aborted
kernel: scsi0: Resetting BusLogic BT-958 due to Target 0
kernel: scsi0: *** BusLogic BT-958 Initialized Successfully ***
Most often, disk failures look like these,
kernel: sidisk I/O error: dev 08:01, sector 1590410
kernel: SCSI disk error : host 0 channel 0 id 0 lun 0 return code = 28000002
or these
kernel: hde: read_intr: error=0x10 { SectorIdNotFound }, CHS=31563/14/35, sector=0
kernel: hde: read_intr: status=0x59 { DriveReady SeekComplete DataRequest Error }
And, as expected, the classic /proc/mdstat look will also reveal problems,
Personalities : [linear] [raid0] [raid1] [translucent]
read_ahead not set
md7 : active raid1 sdc9[0] sdd5[8] 32000 blocks [2/1] [U_]
Later on this section we will learn how to monitor RAID with mdadm so
we can receive alert reports about disk failures. Now it's time to
learn more about /proc/mdstat interpretation.
Querying the array status
You can always take a look at the array status by doing cat /proc/mdstat It won't hurt. Take a look at the /proc/mdstat page to learn how to read the file.
Finally, remember that you can also use mdadm to check the arrays out.
mdadm --detail /dev/mdx
These commands will show spare and failed disks loud and clear.
Simulating a drive failure
If you plan to use RAID to get fault-tolerance, you may also want to test your setup, to see if it really works. Now, how does one simulate a disk failure?
The short story is, that you can't, except perhaps for putting a fire axe thru the drive you want to "simulate" the fault on. You can never know what will happen if a drive dies. It may electrically take the bus it is attached to with it, rendering all drives on that bus inaccessible. The drive may also just report a read/write fault to the SCSI/IDE/SATA layer, which, if done properly, in turn makes the RAID layer handle this situation gracefully. This is fortunately the way things often go.
Remember, that you must be running RAID-{1,4,5,6,10} for your array to be able to survive a disk failure. Linear- or RAID-0 will fail completely when a device is missing.
Force-fail by hardware
If you want to simulate a drive failure, you can just plug out the drive. You should do this with the power off. If you are interested in testing whether your data can survive with a disk less than the usual number, there is no point in being a hot-plug cowboy here. Take the system down, unplug the disk, and boot it up again.
Look in the syslog, and look at /proc/mdstat to see how the RAID is doing. Did it work? Did you get an email from the mdadm monitor?
Faulty disks should appear marked with an (F) if you look at /proc/mdstat. Also, users of mdadm should see the device state as faulty.
When you've re-connected the disk again (with the power off, of course, remember), you can add the "new" device to the RAID again, with the mdadm --add' command.
Force-fail by software
You can just simulate a drive failure without unplugging things. Just running the command
mdadm --manage --set-faulty /dev/md1 /dev/sdc2
should be enough to fail the disk /dev/sdc2 of the array /dev/md1.
Now things move up and fun appears. First, you should see something
like the first line of this on your system's log. Something like the
second line will appear if you have spare disks configured.
kernel: raid1: Disk failure on sdc2, disabling device.
kernel: md1: resyncing spare disk sdb7 to replace failed disk
Checking /proc/mdstat out will show the degraded array. If there was a
spare disk available, reconstruction should have started.
Another useful command at this point is:
mdadm --detail /dev/md1
Enjoy the view.
Now you've seen how it goes when a device fails. Let's fix things up.
First, we will remove the failed disk from the array. Run the command
mdadm /dev/md1 -r /dev/sdc2
Note that mdadm cannot pull a disk out of a running array. For obvious reasons, only faulty disks can be hot-removed from an array (even stopping and unmounting the device won't help - if you ever want to remove a 'good' disk, you have to tell the array to put it into the 'failed' state as above).
Now we have a /dev/md1 which has just lost a device. This could be a degraded RAID or perhaps a system in the middle of a reconstruction process. We wait until recovery ends before setting things back to normal.
So the trip ends when we send /dev/sdc2 back home.
mdadm /dev/md1 -a /dev/sdc2
As the prodigal son returns to the array, we'll see it becoming an
active member of /dev/md1 if necessary. If not, it will be marked as
a spare disk. That's management made easy.
Simulating data corruption
RAID (be it hardware or software), assumes that if a write to a disk doesn't return an error, then the write was successful. Therefore, if your disk corrupts data without returning an error, your data will become corrupted. This is of course very unlikely to happen, but it is possible, and it would result in a corrupt filesystem.
RAID cannot, and is not supposed to, guard against data corruption on the media. Therefore, it doesn't make any sense either, to purposely corrupt data (using dd for example) on a disk to see how the RAID system will handle that. It is most likely (unless you corrupt the RAID superblock) that the RAID layer will never find out about the corruption, but your filesystem on the RAID device will be corrupted.
This is the way things are supposed to work. RAID is not a guarantee for data integrity, it just allows you to keep your data if a disk dies (that is, with RAID levels above or equal one, of course).
Monitoring RAID arrays
You can run mdadm as a daemon by using the follow-monitor mode. If needed, that will make mdadm send email alerts to the system administrator when arrays encounter errors or fail. Also, follow mode can be used to trigger contingency commands if a disk fails, like giving a second chance to a failed disk by removing and reinserting it, so a non-fatal failure could be automatically solved.
Let's see a basic example. Running
mdadm --monitor --mail=root@localhost --delay=1800 /dev/md2
should release a mdadm daemon to monitor /dev/md2. The delay parame- ter means that polling will be done in intervals of 1800 seconds. Finally, critical events and fatal errors should be e-mailed to the system manager. That's RAID monitoring made easy.
Finally, the --program or --alert parameters specify the program to be run whenever an event is detected.
Note that the mdadm daemon will never exit once it decides that there are arrays to monitor, so it should normally be run in the background. Remember that your are running a daemon, not a shell command.
Using mdadm to monitor a RAID array is simple and effective. However, there are fundamental problems with that kind of monitoring - what happens, for example, if the mdadm daemon stops? In order to overcome this problem, one should look towards "real" monitoring solutions. There are a number of free software, open source, and even commercial solutions available which can be used for Software RAID monitoring on Linux. A search on FreshMeat should return a good number of matches.
Tweaking, tuning and troubleshooting
Autodetection
Autodetection is a now-deprecated way to allow the RAID devices to be automatically recognized by the kernel at boot-time, right after the ordinary partition detection is done. If your system still uses autodetect then here
This requires several things:
1. You need autodetection support in the kernel. Check this
1. You must be using version 0.9 superblocks (non-persistent or 1.x won't work).
1. The partition-types of the devices used in the RAID must be set to 0xFD (use fdisk and set the type to "fd")
NOTE: Be sure that your RAID is NOT RUNNING before changing the
partition types. Use mdadm --stop /dev/md0 to stop the device.
If you set up 1, 2 and 3 from above, autodetection should be set up. Try rebooting. When the system comes up, cat'ing /proc/mdstat should tell you that your RAID is running.
During boot, you could see messages similar to these:
Oct 22 00:51:59 malthe kernel: SCSI device sdg: hdwr sector= 512 bytes. Sectors= 12657717 [6180 MB] [6.2 GB] Oct 22 00:51:59 malthe kernel: Partition check: Oct 22 00:51:59 malthe kernel: sda: sda1 sda2 sda3 sda4 Oct 22 00:51:59 malthe kernel: sdb: sdb1 sdb2 Oct 22 00:51:59 malthe kernel: sdc: sdc1 sdc2 Oct 22 00:51:59 malthe kernel: sdd: sdd1 sdd2 Oct 22 00:51:59 malthe kernel: sde: sde1 sde2 Oct 22 00:51:59 malthe kernel: sdf: sdf1 sdf2 Oct 22 00:51:59 malthe kernel: sdg: sdg1 sdg2 Oct 22 00:51:59 malthe kernel: autodetecting RAID arrays Oct 22 00:51:59 malthe kernel: (read) sdb1's sb offset: 6199872 Oct 22 00:51:59 malthe kernel: bind<sdb1,1> Oct 22 00:51:59 malthe kernel: (read) sdc1's sb offset: 6199872 Oct 22 00:51:59 malthe kernel: bind<sdc1,2> Oct 22 00:51:59 malthe kernel: (read) sdd1's sb offset: 6199872 Oct 22 00:51:59 malthe kernel: bind<sdd1,3> Oct 22 00:51:59 malthe kernel: (read) sde1's sb offset: 6199872 Oct 22 00:51:59 malthe kernel: bind<sde1,4> Oct 22 00:51:59 malthe kernel: (read) sdf1's sb offset: 6205376 Oct 22 00:51:59 malthe kernel: bind<sdf1,5> Oct 22 00:51:59 malthe kernel: (read) sdg1's sb offset: 6205376 Oct 22 00:51:59 malthe kernel: bind<sdg1,6> Oct 22 00:51:59 malthe kernel: autorunning md0 Oct 22 00:51:59 malthe kernel: running: <sdg1><sdf1><sde1><sdd1><sdc1><sdb1> Oct 22 00:51:59 malthe kernel: now! Oct 22 00:51:59 malthe kernel: md: md0: raid array is not clean -- starting background reconstruction
This is output from the autodetection of a RAID-5 array that was not
cleanly shut down (eg. the machine crashed). Reconstruction is auto-
matically initiated. Mounting this device is perfectly safe, since
reconstruction is transparent and all data are consistent (it's only
the parity information that is inconsistent - but that isn't needed
until a device fails).
Autostarted devices are also automatically stopped at shutdown. Don't worry about init scripts. Just use the /dev/md devices as any other /dev/sd or /dev/hd devices.
Yes, it really is that easy - but it is also full of problems and should be avoided.
Booting on RAID
There are several ways to set up a system that mounts its root filesystem on a RAID device. Some distributions allow for RAID setup in the installation process, and this is by far the easiest way to get a nicely set up RAID system.
Newer LILO distributions can handle RAID-1 devices, and thus the kernel can be loaded at boot-time from a RAID device. If configured appropriately LILO will correctly write boot-records on all disks in the array, to allow booting even if the primary disk fails (default LILO configurations are generally not setup like this).
If you are using grub instead of LILO, then just start grub and configure it to use the second (or third, or fourth...) disk in the RAID-1 array you want to boot off as its root device and run setup. And that's all.
For example, on an array consisting of /dev/hda1 and /dev/hdc1 where both partitions should be bootable you should just do this:
grub grub>device (hd0) /dev/hdc grub>root (hd0,0) grub>setup (hd0)
Some users have experienced problems with this, reporting that although booting with one drive connected worked, booting with both two drives failed. Nevertheless, running the described procedure with both disks fixed the problem, allowing the system to boot from either single drive or from the RAID-1
Another way of ensuring that your system can always boot is, to create a boot floppy when all the setup is done. If the disk on which the /boot filesystem resides dies, you can always boot from the floppy. On RedHat and RedHat derived systems, this can be accomplished with the mkbootdisk command.
Root filesystem on RAID
In order to have a system booting on RAID, the root filesystem (/) must be mounted on a RAID device. Two methods for achieving this is supplied bellow. The methods below assume that you install on a normal partition, and then - when the installation is complete - move the contents of your non-RAID root filesystem onto a new RAID device. Please not that this is no longer needed in general, as most newer GNU/Linux distributions support installation on RAID devices (and creation of the RAID devices during the installation process). However, you may still want to use the methods below, if you are migrating an existing system to RAID.
Method 1
This method assumes you have a spare disk you can install the system on, which is not part of the RAID you will be configuring.
- First, install a normal system on your extra disk.
- Get the kernel you plan on running, get the raid-patches and the tools, and make your system boot with this new RAID-aware kernel. Make sure that RAID-support is in the kernel, and is not loaded as modules.
- Ok, now you should configure and create the RAID you plan to use for the root filesystem. This is standard procedure, as described elsewhere in this document.
- Just to make sure everything's fine, try rebooting the system to see if the new RAID comes up on boot. It should.
- Put a filesystem on the new array (using mke2fs), and mount it under /mnt/newroot
- Now, copy the contents of your current root-filesystem (the spare disk) to the new root-filesystem (the array). There are lots of ways to do this, one of them is
cd /
find . -xdev | cpio -pm /mnt/newroot
another way to copy everything from / to /mnt/newroot could be
cp -ax / /mnt/newroot
- You should modify the /mnt/newroot/etc/fstab file to use the correct device (the /dev/md? root device) for the root filesystem.
- Now, unmount the current /boot filesystem, and mount the boot device on /mnt/newroot/boot instead. This is required for LILO to run successfully in the next step.
- Update /mnt/newroot/etc/lilo.conf to point to the right devices. The boot device must still be a regular disk (non-RAID device), but the root device should point to your new RAID. When done, run
lilo -r /mnt/newroot
complete with no errors.
- Reboot the system, and watch everything come up as expected :)
If you're doing this with IDE disks, be sure to tell your BIOS that all disks are "auto-detect" types, so that the BIOS will allow your machine to boot even when a disk is missing.
Method 2
This method requires that your kernel and raidtools understand the failed-disk directive in the /etc/raidtab file - if you are working on a really old system this may not be the case, and you will need to upgrade your tools and/or kernel first.
You can only use this method on RAID levels 1 and above, as the method uses an array in "degraded mode" which in turn is only possible if the RAID level has redundancy. The idea is to install a system on a disk which is purposely marked as failed in the RAID, then copy the system to the RAID which will be running in degraded mode, and finally making the RAID use the no-longer needed "install-disk", zapping the old installation but making the RAID run in non-degraded mode.
- First, install a normal system on one disk (that will later become part of your RAID). It is important that this disk (or partition) is not the smallest one. If it is, it will not be possible to add it to the RAID later on!
- Then, get the kernel, the patches, the tools etc. etc. You know the drill. Make your system boot with a new kernel that has the RAID support you need, compiled into the kernel.
- Now, set up the RAID with your current root-device as the failed-disk in the /etc/raidtab file. Don't put the failed-disk as the first disk in the raidtab, that will give you problems with starting the RAID. Create the RAID, and put a filesystem on it. If using mdadm, you can create a degraded array just by running something like
mdadm -C /dev/md0 --level raid1 --raid-disks 2 missing /dev/hdc1
note the missing parameter.
- Try rebooting and see if the RAID comes up as it should
- Copy the system files, and reconfigure the system to use the RAID as root-device, as described in the previous section.
- When your system successfully boots from the RAID, you can modify the /etc/raidtab file to include the previously failed-disk as a normal raid-disk. Now, raidhotadd the disk to your RAID.
- You should now have a system that can boot from a non-degraded RAID.
Making the system boot on RAID
For the kernel to be able to mount the root filesystem, all support for the device on which the root filesystem resides, must be present in the kernel. Therefore, in order to mount the root filesystem on a RAID device, the kernel must have RAID support.
The normal way of ensuring that the kernel can see the RAID device is to simply compile a kernel with all necessary RAID support compiled in. Make sure that you compile the RAID support into the kernel, and not as loadable modules. The kernel cannot load a module (from the root filesystem) before the root filesystem is mounted.
However, since RedHat-6.0 ships with a kernel that has new-style RAID support as modules, I here describe how one can use the standard RedHat-6.0 kernel and still have the system boot on RAID.
Booting with RAID as module
You will have to instruct LILO to use a RAM-disk in order to achieve this. Use the mkinitrd command to create a ramdisk containing all kernel modules needed to mount the root partition. This can be done as:
mkinitrd --with=<module> <ramdisk name> <kernel>
For example:
mkinitrd --preload raid5 --with=raid5 raid-ramdisk 2.2.5-22
This will ensure that the specified RAID module is present at boot- time, for the kernel to use when mounting the root device.
Modular RAID on Debian GNU/Linux after move to RAID
Debian users may encounter problems using an initrd to mount their root filesystem from RAID, if they have migrated a standard non-RAID Debian install to root on RAID.
If your system fails to mount the root filesystem on boot (you will see this in a "kernel panic" message), then the problem may be that the initrd filesystem does not have the necessary support to mount the root filesystem from RAID.
Debian seems to produce its initrd.img files on the assumption that the root filesystem to be mounted is the current one. This will usually result in a kernel panic if the root filesystem is moved to the raid device and you attempt to boot from that device using the same initrd image. The solution is to use the mkinitrd command but specifying the proposed new root filesystem. For example, the following commands should create and set up the new initrd on a Debian system:
% mkinitrd -r /dev/md0 -o /boot/initrd.img-2.4.22raid % mv /initrd.img /initrd.img-nonraid % ln -s /boot/initrd.img-raid /initrd.img"
Converting a non-RAID RedHat System to run on Software RAID
This section was written and contributed by Mark Price, IBM. The text has undergone minor changes since his original work.
Notice: the following information is provided "AS IS" with no representation or warranty of any kind either express or implied. You may use it freely at your own risk, and no one else will be liable for any damages arising out of such usage.
Introduction
The technote details how to convert a linux system with non RAID devices to run with a Software RAID configuration.
Scope
This scenario was tested with Redhat 7.1, but should be applicable to any release which supports Software RAID (md) devices.
Pre-conversion example system
The test system contains two SCSI disks, sda and sdb both of of which are the same physical size. As part of the test setup, I configured both disks to have the same partition layout, using fdisk to ensure the number of blocks for each partition was identical.
DEVICE MOUNTPOINT SIZE DEVICE MOUNTPOINT SIZE /dev/sda1 / 2048MB /dev/sdb1 2048MB /dev/sda2 /boot 80MB /dev/sdb2 80MB /dev/sda3 /var/ 100MB /dev/sdb3 100MB /dev/sda4 SWAP 1024MB /dev/sdb4 SWAP 1024MB
In our basic example, we are going to set up a simple RAID-1 Mirror,
which requires only two physical disks.
Step-1 - boot rescue cd/floppy
The redhat installation CD provides a rescue mode which boots into linux from the CD and mounts any filesystems it can find on your disks.
At the lilo prompt type
lilo: linux rescue
With the setup described above, the installer may ask you which disk your root filesystem in on, either sda or sdb. Select sda.
The installer will mount your filesytems in the following way.
DEVICE MOUNTPOINT TEMPORARY MOUNT POINT /dev/sda1 / /mnt/sysimage /dev/sda2 /boot /mnt/sysimage/boot /dev/sda3 /var /mnt/sysimage/var /dev/sda6 /home /mnt/sysimage/home
Note: - Please bear in mind other distributions may mount your
filesystems on different mount points, or may require you to mount
them by hand.
Step-2 - create a /etc/raidtab file
Create the file /mnt/sysimage/etc/raidtab (or wherever your real /etc file system has been mounted.
For our test system, the raidtab file would like like this.
raiddev /dev/md0
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
chunk-size 4
persistent-superblock 1
device /dev/sda1
raid-disk 0
device /dev/sdb1
raid-disk 1
raiddev /dev/md1
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
chunk-size 4
persistent-superblock 1
device /dev/sda2
raid-disk 0
device /dev/sdb2
raid-disk 1
raiddev /dev/md2
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
chunk-size 4
persistent-superblock 1
device /dev/sda3
raid-disk 0
device /dev/sdb3
raid-disk 1
Note: - It is important that the devices are in the correct order. ie. that /dev/sda1 is raid-disk 0 and not raid-disk 1. This instructs the md driver to sync from /dev/sda1, if it were the other way around it would sync from /dev/sdb1 which would destroy your filesystem.
Now copy the raidtab file from your real root filesystem to the current root filesystem.
(rescue)# cp /mnt/sysimage/etc/raidtab /etc/raidtab
Step-3 - create the md devices
There are two ways to do this, copy the device files from /mnt/sysimage/dev or use mknod to create them. The md device, is a (b)lock device with major number 9.
(rescue)# mknod /dev/md0 b 9 0 (rescue)# mknod /dev/md1 b 9 1 (rescue)# mknod /dev/md2 b 9 2
Step-4 - unmount filesystems
In order to start the raid devices, and sync the drives, it is necessary to unmount all the temporary filesystems.
(rescue)# umount /mnt/sysimage/var (rescue)# umount /mnt/sysimage/boot (rescue)# umount /mnt/sysimage/proc (rescue)# umount /mnt/sysimage
Please note, you may not be able to umount /mnt/sysimage. This problem
can be caused by the rescue system - if you choose to manually mount
your filesystems instead of letting the rescue system do this automat-
ically, this problem should go away.
Step-5 - start raid devices
Because there are filesystems on /dev/sda1, /dev/sda2 and /dev/sda3 it is necessary to force the start of the raid device.
(rescue)# mkraid --really-force /dev/md2
You can check the completion progress by cat'ing the /proc/mdstat file. It shows you status of the raid device and percentage left to sync.
Continue with /boot and /
(rescue)# mkraid --really-force /dev/md1 (rescue)# mkraid --really-force /dev/md0
he md driver syncs one device at a time.
Step-6 - remount filesystems
Mount the newly synced filesystems back into the /mnt/sysimage mount points.
(rescue)# mount /dev/md0 /mnt/sysimage (rescue)# mount /dev/md1 /mnt/sysimage/boot (rescue)# mount /dev/md2 /mnt/sysimage/var
Step-7 - change root
You now need to change your current root directory to your real root file system.
(rescue)# chroot /mnt/sysimage
Step-8 - edit config files
You need to configure lilo and /etc/fstab appropriately to boot from and mount the md devices.
Note: - The boot device MUST be a non-raided device. The root device is your new md0 device. eg.
boot=/dev/sda map=/boot/map install=/boot/boot.b prompt timeout=50 message=/boot/message linear default=linux
image=/boot/vmlinuz
label=linux
read-only
root=/dev/md0
Alter /etc/fstab
/dev/md0 / ext3 defaults 1 1 /dev/md1 /boot ext3 defaults 1 2 /dev/md2 /var ext3 defaults 1 2 /dev/sda4 swap swap defaults 0 0
Step-9 - run LILO
With the /etc/lilo.conf edited to reflect the new root=/dev/md0 and with /dev/md1 mounted as /boot, we can now run /sbin/lilo -v on the chrooted filesystem.
Step-10 - change partition types
The partition types of the all the partitions on ALL Drives which are used by the md driver must be changed to type 0xFD.
Use fdisk to change the partition type, using option 't'.
(rescue)# fdisk /dev/sda (rescue)# fdisk /dev/sdb
Use the 'w' option after changing all the required partitions to save the partion table to disk.
Step-11 - resize filesystem
When we created the raid device, the physical partion became slightly smaller because a second superblock is stored at the end of the partition. If you reboot the system now, the reboot will fail with an error indicating the superblock is corrupt.
Resize them prior to the reboot, ensure that the all md based filesystems are unmounted except root, and remount root read-only.
(rescue)# mount / -o remount,ro
You will be required to fsck each of the md devices. This is the reason for remounting root read-only. The -f flag is required to force fsck to check a clean filesystem.
(rescue)# e2fsck -f /dev/md0
This will generate the same error about inconsistent sizes and possibly corrupted superblock.Say N to 'Abort?'.
(rescue)# resize2fs /dev/md0
Repeat for all /dev/md devices.
Step-12 - checklist
The next step is to reboot the system, prior to doing this run through the checklist below and ensure all tasks have been completed.
- All devices have finished syncing. Check /proc/mdstat
- /etc/fstab has been edited to reflect the changes to the device names.
- /etc/lilo.conf has beeb edited to reflect root device change.
- /sbin/lilo has been run to update the boot loader.
- The kernel has both SCSI and RAID(MD) drivers built into the kernel.
- The partition types of all partitions on disks that are part of an md device have been changed to 0xfd.
- The filesystems have been fsck'd and resize2fs'd.
Step-13 - reboot
You can now safely reboot the system, when the system comes up it will auto discover the md devices (based on the partition types).
Your root filesystem will now be mirrored.
Sharing spare disks between different arrays
When running mdadm in the follow/monitor mode you can make different arrays share spare disks. That will surely make you save storage space without losing the comfort of fallback disks.
In the world of software RAID, this is a brand new never-seen-before feature: for securing things to the point of spare disk areas, you just have to provide one single idle disk for a bunch of arrays.
With mdadm is running as a daemon, you have an agent polling arrays at regular intervals. Then, as a disk fails on an array without a spare disk, mdadm removes an available spare disk from another array and inserts it into the array with the failed disk. The reconstruction process begins now in the degraded array as usual.
To declare shared spare disks, just use the spare-group parameter when invoking mdadm as a daemon.
Pitfalls
Never NEVER never re-partition disks that are part of a running RAID. If you must alter the partition table on a disk which is a part of a RAID, stop the array first, then repartition.
It is easy to put too many disks on a bus. A normal Fast-Wide SCSI bus can sustain 10 MB/s which is less than many disks can do alone today. Putting six such disks on the bus will of course not give you the expected performance boost. It is becoming equally easy to saturate the PCI bus - remember, a normal 32-bit 33 MHz PCI bus has a theoretical maximum bandwidth of around 133 MB/sec, considering command overhead etc. you will see a somewhat lower real-world transfer rate. Some disks today has a throughput in excess of 30 MB/sec, so just four of those disks will actually max out your PCI bus! When designing high-performance RAID systems, be sure to take the whole I/O path into consideration - there are boards with more PCI busses, with 64-bit and 66 MHz busses, and with PCI-X.
More SCSI controllers will only give you extra performance, if the SCSI busses are nearly maxed out by the disks on them. You will not see a performance improvement from using two 2940s with two old SCSI disks, instead of just running the two disks on one controller.
If you forget the persistent-superblock option, your array may not start up willingly after it has been stopped. Just re-create the array with the option set correctly in the raidtab. Please note that this will destroy the information on the array!
If a RAID-5 fails to reconstruct after a disk was removed and re- inserted, this may be because of the ordering of the devices in the raidtab. Try moving the first "device ..." and "raid-disk ..." pair to the bottom of the array description in the raidtab file. If you have read the rest of this HOWTO, you should already have a pretty good idea about what reconstruction of a degraded RAID involves. Let us summarize:
- Power down the system
- Replace the failed disk
- Power up the system once again.
- Use raidhotadd /dev/mdX /dev/sdX to re-insert the disk in the array
- Have coffee while you watch the automatic reconstruction running
In some cases, the "good" disk does not have a boot block, as might happen if the degraded disk is the "first" one, e.g. the hda or sda device. In this case you might not be able to boot the system. Try to reconstruct the MBR with the boot loader of choice. The installation disk of your linux distro might have a rescue mode and assist you in this task.
And that's it.
Well, it usually is, unless you're unlucky and your RAID has been rendered unusable because more disks than the ones redundant failed. This can actually happen if a number of disks reside on the same bus, and one disk takes the bus with it as it crashes. The other disks, however fine, will be unreachable to the RAID layer, because the bus is down, and they will be marked as faulty. On a RAID-5 where you can spare one disk only, loosing two or more disks can be fatal.
Recovery from a multiple disk failure
This section is the explanation that Martin Bene gave to me, and describes a possible recovery from the scary scenario outlined above. It involves using the failed-disk directive in your /etc/raidtab (so for people running patched 2.2 kernels, this will only work on kernels 2.2.10 and later).
The scenario is:
- A controller dies and takes two disks offline at the same time,
- All disks on one scsi bus can no longer be reached if a disk dies,
- A cable comes loose...
In short: quite often you get a temporary failure of several disks at once; afterwards the RAID superblocks are out of sync and you can no longer init your RAID array.
If using mdadm, you could first try to run:
mdadm --assemble --force
If not, there's one thing left: rewrite the RAID superblocks by mkraid --force
To get this to work, you'll need to have an up to date /etc/raidtab - if it doesn't EXACTLY match devices and ordering of the original disks this will not work as expected, but will most likely completely obliterate whatever data you used to have on your disks.
Look at the sylog produced by trying to start the array, you'll see the event count for each superblock; usually it's best to leave out the disk with the lowest event count, i.e the oldest one.
If you mkraid without failed-disk, the recovery thread will kick in immediately and start rebuilding the parity blocks - not necessarily what you want at that moment.
With failed-disk you can specify exactly which disks you want to be active and perhaps try different combinations for best results. BTW, only mount the filesystem read-only while trying this out... This has been successfully used by at least two guys I've been in contact with.
recovery and resync
The following is a recollection of what Neil Brown and others have written on the linux-raid mailing list.
"resync" and "recovery" are handled very differently in raid10. "check" and "repair" are special cases of "resync".
recovery
The purpose of the recovery process is to fill a new disk with the relevant information from a running array.
The assumption is that all data on the new disk needs to be written, and that the other data on the running array is correct.
"recovery" walks addresses from the start to the end of the component drives. Thus only data for the specific component drive is adressed.
At each address, it considers each drive which is being recovered and finds a place on a different device to read the block for the current (drive,address) from. It schedules a read and when the read request completes it schedules the write.
On an f2 layout, this will read one drive from halfway to the end, then from the start to halfway, and will write the other drive sequentially.
resync
The purpose of resync is to ensure that all data on the array is syncronized.
There is an assumption that most, if not all, of the data is allready OK.
For raid10 "resync" walks the addresses from the start to end of the arra