Linux Software Raid How to Print

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1. Introduction

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

This HOWTO describes the "new-style" RAID present in the 2.4 and 2.6 kernel series only. It does not describe the "old-style" RAID functionality present in 2.0 and 2.2 kernels.

The home site for this HOWTO is, where updated versions appear first. The howto was originally written by Jakob stergaard based on a large number of emails between the author and Ingo Molnar ( -- one of the RAID developers --, the linux-raid mailing list ( and various other people. Emilio Bueso ( co-wrote the 1.0 version.

If you want to use the new-style RAID with 2.0 or 2.2 kernels, you should get a patch for your kernel, from The standard 2.2 kernels does not have direct support for the new-style RAID described in this HOWTO. Therefore these patches are needed. The old-style RAID support in standard 2.0 and 2.2 kernels is buggy and lacks several important features present in the new-style RAID software.

Some of the information in this HOWTO may seem trivial, if you know RAID all ready. Just skip those parts.

1.1 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.

1.2 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.

1.3 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".

1.4 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.
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 4 kB to disk 0, 4 kB to disk 1, 4 kB to disk 2, then 4 kB to disk 0 again, and so on. 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.
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 6 ms equals a read of 123 kB at 20 MB/sec), picking the disk that will have the shortest seek time does actually give a noticeable performance improvement.
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.
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 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.
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, 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 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), or similar to RAID-1 writes. 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.

1.5 Requirements

This HOWTO assumes you are using Linux 2.4 or later. However, it is possible to use Software RAID in late 2.2.x or 2.0.x Linux kernels with a matching RAID patch and the 0.90 version of the raidtools. Both the patches and the tools can be found at The RAID patch, the raidtools package, and the kernel should all match as close as possible. At times it can be necessary to use older kernels if raid patches are not available for the latest kernel.

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 the raidtools for your kernel.

2. Why RAID?

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

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 substitute for good backups. Some RAID levels will make your systems immune to data loss from single-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 (single disk failure). It is not in itself a complete data safety solution. This is very important to realize.

2.1 Device and filesystem support

Linux RAID can work on most block devices. It doesn't matter whether you use IDE or SCSI devices, or a mixture. Some people have also used the Network Block Device (NBD) with more or less 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.

2.2 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 Chapter one for a mention of the performance characteristics of each level.

2.3 Swapping on RAID

There's no reason to use RAID 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 looks 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 RAID, since this has been a kernel feature for a long time.

Another 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. But if the system has been swapping on the now faulty device, you will for sure be going down. Swapping on a RAID-1 device 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.

2.4 Why mdadm?

The classic raidtools are the standard software RAID management tool for Linux, so using mdadm is not a must.

However, if you find raidtools cumbersome or limited, mdadm (multiple devices admin) is an extremely useful tool for running RAID systems. It can be used as a replacement for the raidtools, or as a supplement.

The mdadm tool, written by Neil Brown, a software engineer at the University of New South Wales and a kernel developer, is now at version 1.4.0 and has proved to be quite stable. There is much positive response on the Linux-raid mailing list and mdadm is likely to become widespread in the future.

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

3. Devices

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

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".

3.1 Spare disks

Spare disks 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 "bad" 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 all redundancy is preserved by means of the spare disk.

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 will lead to a complete RAID failure. 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.

3.2 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.

Faulty disks still appear and behave as members of the array. The RAID layer just treats crashed devices as inactive parts of the filesystem.


4. Hardware issues

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

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 bus. Running disks as master/slave is horrible for performance. IDE is really bad at accessing more that one drive per bus. Of Course, all 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.

4.1 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.

4.2 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 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
Remove the drive to replace from the array:
raidhotremove /dev/md0 /dev/sdb1
Look up the Host, Channel, ID and Lun of the drive to replace, by looking in
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
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:
raidhotadd /dev/md0 /dev/sdb2

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.


5. RAID setup

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

5.1 General setup

This is what you need for any of the RAID levels:

A kernel. Preferably a kernel from the 2.4 series. Alternatively a 2.0 or 2.2 kernel with the RAID patches applied.
The RAID tools.
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.

Create the partitions you want to include in your RAID set.

5.2 Downloading and installing the RAID tools

The RAID tools are included in almost every major Linux distribution.

IMPORTANT: If using Debian Woody (3.0) or later, you can install the package by running

apt-get install raidtools2
This raidtools2 is a modern version of the old raidtools package, which doesn't support the persistent-superblock and parity-algorithm settings.


5.3 Downloading and installing mdadm

You can download the most recent mdadm tarball at Issue a nice make install to compile and then install mdadm and its documentation, manual pages and example files.

tar xvf ./mdadm-1.4.0.tgz
cd mdadm-1.4.0.tgz
make install
If using an RPM-based distribution, you can download and install the package file found at

rpm -ihv mdadm-1.4.0-1.i386.rpm
If using Debian Woody (3.0) or later, you can install the package by running

apt-get install mdadm
Gentoo has this package available in the portage tree. There you can run

emerge mdadm
Other distributions may also have this package available. Now, let's go mode-specific.


5.4 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.

Set up the /etc/raidtab file to describe your setup. I set up a raidtab for two disks in linear mode, and the file looked like this:


raiddev /dev/md0
raid-level linear
nr-raid-disks 2
chunk-size 32
persistent-superblock 1
device /dev/sdb6
raid-disk 0
device /dev/sdc5
raid-disk 1
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.

Ok, let's create the array. Run the command

mkraid /dev/md0

This will initialize your array, write the persistent superblocks, and start the array.

If you are 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 output 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.

5.5 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.

Set up the /etc/raidtab file to describe your configuration. An example raidtab looks like:

raiddev /dev/md0
raid-level 0
nr-raid-disks 2
persistent-superblock 1
chunk-size 4
device /dev/sdb6
raid-disk 0
device /dev/sdc5
raid-disk 1
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.


Again, you just run

mkraid /dev/md0
to initialize the array. This should initialize the superblocks and start 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.

5.6 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.

Set up the /etc/raidtab file like this:

raiddev /dev/md0
raid-level 1
nr-raid-disks 2
nr-spare-disks 0
persistent-superblock 1
device /dev/sdb6
raid-disk 0
device /dev/sdc5
raid-disk 1
If you have spare disks, you can add them to the end of the device specification like

device /dev/sdd5
spare-disk 0
Remember to set the nr-spare-disks entry correspondingly.


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.

Issue the

mkraid /dev/md0
command to begin the mirror initialization.


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.

5.7 RAID-4

Note! I haven't tested this setup myself. The setup below is my best guess, not something I have actually had up running. If you use RAID-4, please write to the author and share your experiences.

You have three or more devices of roughly the same size, one device is significantly faster than the other devices, and you want to combine them all into one larger device, still maintaining some redundancy information. Eventually you have a number of devices you wish to use as spare-disks.

Set up the /etc/raidtab file like this:

raiddev /dev/md0
raid-level 4
nr-raid-disks 4
nr-spare-disks 0
persistent-superblock 1
chunk-size 32
device /dev/sdb1
raid-disk 0
device /dev/sdc1
raid-disk 1
device /dev/sdd1
raid-disk 2
device /dev/sde1
raid-disk 3
If we had any spare disks, they would be inserted in a similar way, following the raid-disk specifications;

device /dev/sdf1
spare-disk 0
as usual.


Your array can be initialized with the

mkraid /dev/md0
command as usual.


You should see the section on special options for mke2fs before formatting the device.

5.8 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.

Set up the /etc/raidtab file like this:

raiddev /dev/md0
raid-level 5
nr-raid-disks 7
nr-spare-disks 0
persistent-superblock 1
parity-algorithm left-symmetric
chunk-size 32
device /dev/sda3
raid-disk 0
device /dev/sdb1
raid-disk 1
device /dev/sdc1
raid-disk 2
device /dev/sdd1
raid-disk 3
device /dev/sde1
raid-disk 4
device /dev/sdf1
raid-disk 5
device /dev/sdg1
raid-disk 6
If we had any spare disks, they would be inserted in a similar way, following the raid-disk specifications;

device /dev/sdh1
spare-disk 0
And so on.


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.

Ok, enough talking. You set up the /etc/raidtab, so let's see if it works. Run the

mkraid /dev/md0
command, 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.

Ok, now when you have your RAID device running, you can always stop it or re-start it using the

raidstop /dev/md0

raidstart /dev/md0


With mdadm you can stop the device using

mdadm -S /dev/md0
and re-start it with

mdadm -R /dev/md0
Instead of putting these into init-files and rebooting a zillion times to make that work, read on, and get autodetection running.


5.9 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 is 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 initialized with the persistent-superblock option in the /etc/raidtab file, a special superblock is written in the beginning of 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.

You should however still maintain a consistent /etc/raidtab file, since you may need this file for later reconstruction 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 Autodetection section.

5.10 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 value, 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".


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

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.


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.


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.


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.

5.11 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} 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.


6. Detecting, querying and testing

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

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.

6.1 Detecting a drive failure

No mistery here. It's enough with a quick look to the standard log and stat files to notice 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.


6.2 Querying the arrays status

You can always take a look at /proc/mdstat. It won't hurt. Let's learn how to read the file. For example,

Personalities : [raid1]
read_ahead 1024 sectors
md5 : active raid1 sdb5[1] sda5[0]
4200896 blocks [2/2] [UU]

md6 : active raid1 sdb6[1] sda6[0]
2104384 blocks [2/2] [UU]

md7 : active raid1 sdb7[1] sda7[0]
2104384 blocks [2/2] [UU]

md2 : active raid1 sdc7[1] sdd8[2] sde5[0]
1052160 blocks [2/2] [UU]

unused devices: none
To identify the spare devices, first look for the [#/#] value on a line. The first number is the number of a complete raid device as defined. Lets say it is "n". The raid role numbers [#] following each device indicate its role, or function, within the raid set. Any device with "n" or higher are spare disks. 0,1,..,n-1 are for the working array.


Also, if you have a failure, the failed device will be marked with (F) after the [#]. The spare that replaces this device will be the device with the lowest role number n or higher that is not marked (F). Once the resync operation is complete, the device's role numbers are swapped.

The order in which the devices appear in the /proc/mdstat output means nothing.

Finally, remember that you can always use raidtools or mdadm to check the arrays out.

mdadm --detail /dev/mdx
lsraid -a /dev/mdx
These commands will show spare and failed disks loud and clear.


6.3 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. I have never heard of that happening though, but it is entirely possible. The drive may also just report a read/write fault to the SCSI/IDE layer, which 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} 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?

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 raidhotadd command.

Force-fail by software

Newer versions of raidtools come with a raidsetfaulty command. By using raidsetfaulty you can just simulate a drive failure without unplugging things off.

Just running the command

raidsetfaulty /dev/md1 /dev/sdc2
should be enough to fail the disk /dev/sdc2 of the array /dev/md1. If you are using mdadm, just type

mdadm --manage --set-faulty /dev/md1 /dev/sdc2
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 fresh utility in newest raidtools is lsraid. Try with

lsraid -a /dev/md1
users of mdadm can run the command

mdadm --detail /dev/md1
and 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

raidhotremove /dev/md1 /dev/sdc2
users of mdadm can run the command

mdadm /dev/md1 -r /dev/sdc2
Note that raidhotremove cannot pull a disk out of a running array. For obvious reasons, only crashed disks are to be hotremoved from an array (running raidstop and unmounting the device won't help).


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.

raidhotadd /dev/md1 /dev/sdc2
As usual, you can use mdadm instead of raidtools. This should be the command

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 an spare disk. That's management made easy.


6.4 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).

6.5 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 parameter 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 is a number of free software, open source, and commercial solutions available which can be used for Software RAID monitoring on Linux. A search on FreshMeat should return a good number of matches.


7. Tweaking, tuning and troubleshooting

This HOWTO is deprecated; the Linux RAID HOWTO is maintained as a wiki by the linux-raid community at

7.1 raid-level

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