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
http://unthought.net/Software-RAID.HOWTO/, where updated
versions appear first. The howto was originally written by Jakob
Østergaard based on a large number of emails between the author
and Ingo Molnar
(firstname.lastname@example.org) -- one of the RAID developers --,
the linux-raid mailing list
(email@example.com) 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
http://people.redhat.com/mingo/ 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.
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.
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.
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".
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
- 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
- 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
- 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.
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
http://people.redhat.com/mingo/. 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.