The Software-RAID HOWTO

The Software-RAID HOWTO
Jakob OEstergaard ([email protected])
v. 0.90.7 19th of January 2000

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 made by Ingo Molnar and others. This is the RAID layer
that will be standard in 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.
______________________________________________________________________

Table of Contents

1. Introduction

1.1 Disclaimer
1.2 Requirements

2. Why RAID ?

2.1 Technicalities
2.2 Terms
2.3 The RAID levels
2.3.1 Spare disks
2.4 Swapping on RAID

3. Hardware issues

3.1 IDE Configuration
3.2 Hot Swap
3.2.1 Hot-swapping IDE drives
3.2.2 Hot-swapping SCSI drives
3.2.3 Hot-swapping with SCA

4. RAID setup

4.1 General setup
4.2 Linear mode
4.3 RAID-0
4.4 RAID-1
4.5 RAID-4
4.6 RAID-5
4.7 The Persistent Superblock
4.8 Chunk sizes
4.8.1 RAID-0
4.8.2 RAID-1
4.8.3 RAID-4
4.8.4 RAID-5
4.9 Options for mke2fs
4.10 Autodetection
4.11 Booting on RAID
4.12 Root filesystem on RAID
4.12.1 Method 1
4.12.2 Method 2
4.13 Making the system boot on RAID
4.13.1 Booting with RAID as module
4.14 Pitfalls

5. Testing

5.1 Simulating a drive failure
5.2 Simulating data corruption

6. Reconstruction

6.1 Recovery from a multiple disk failure

7. Performance

7.1 RAID-0
7.2 RAID-0 with TCQ
7.3 RAID-5
7.4 RAID-10

8. Credits

______________________________________________________________________

1. Introduction

For a description of the older RAID layer, the one which is standard
in 2.0 and 2.2 kernels, see the excellent HOWTO from Linas Vepstas
([email protected]) available from the Linux Documentation Project at
linuxdoc.org.

The home site for this HOWTO is http://ostenfeld.dk/~jakob/Software-
RAID.HOWTO/, where updated versions appear first. The howto is
written by Jakob OEstergaard based on a large number of emails between
the author and Ingo Molnar ([email protected]) -- one of the
RAID developers --, the linux-raid mailing list (linux-
[email protected]) and various other people.

The reason this HOWTO was written even though a Software-RAID HOWTO
already exists is, that the old HOWTO describes the old-style Software
RAID found in the standard 2.0 and 2.2 kernels. This HOWTO describes
the use of the new-style RAID that has been developed more recently.
The new-style RAID has a lot of features not present in old-style
RAID.

If you want to use the new-style RAID with 2.0 or 2.2 kernels, you
should get a patch for your kernel, either from ftp://ftp.[your-
country-code].kernel.org/pub/linux/daemons/raid/alpha, or more
recently 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.

As of this writing, the new-style RAID support is being merged into
the 2.3 development kernels, and will therefore (most likely) be
present in the 2.4 Linux kernel when that one comes out. But until
then, the stable kernels must be patched manually.

You might want to use the -ac kernel releases done by Alan Cox, for
RAID support in 2.2. Some of those contain the new-style RAID, and
that will save you from patching the kernel yourself.

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:

Although RAID seems stable for me, and stable for many other people,
it may not work for you. 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 anyway correct, nor suited for any use
whatsoever. Back up all your data before experimenting with this.
Better safe than sorry.

That said, I must also say that I haven't had a single stability
problem with Software RAID, I use it on quite a few machines with no
problems what so ever, and I haven't seen other people having problems
with random crashes or instability caused by RAID.

1.2. Requirements

This HOWTO assumes you are using a late 2.2.x or 2.0.x kernel with a
matching raid0145 patch and the 0.90 version of the raidtools, or that
you are using a late 2.3 kernel (version > 2.3.46) or eventually 2.4.
Both the patches and the tools can be found at
ftp://ftp.fi.kernel.org/pub/linux/daemons/raid/alpha, and in some
cases 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.

2. 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.

2.1. Technicalities

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.

Be sure that the bus(ses) to the drives are fast enough. You shouldn't
have 14 UW-SCSI drives on one UW bus, if each drive can give 10 MB/s
and the bus can only sustain 40 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.

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. Terms

The word ``RAID'' means ``Linux Software RAID''. This HOWTO does not
treat any aspects of Hardware RAID.

When describing 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.

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''.

2.3. The RAID levels

Here's a short description of what is supported in the Linux RAID
patches. 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. Just skip this section if you know
RAID. Then come back when you are having problems :)

The current RAID patches for Linux supports the following levels:

o Linear mode

o Two or more disks are combined into one physical device. The disks
are ``appended'' to each other, so writing 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 :)

o 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.

o 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.

o RAID-0

o Also called ``stripe'' mode. Like linear mode, except that reads
and writes are done in parallel to the devices. The devices should
have approximately the same size. Since all access is done in
parallel, the devices fill up equally. 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.

o Like linear, there's 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 will probably not be
able to recover much from such a device.

o 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.

o RAID-1

o 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.

o 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.
o Write performance is the slightly worse than on a single device,
because identical copies of the data written must be sent to every
disk in the array. Read performance is usually pretty bad because
of an oversimplified read-balancing strategy in the RAID code.
However, there has been implemented a much improved read-balancing
strategy, which might be available for the Linux-2.2 RAID patches
(ask on the linux-kernel list), and which will most likely be in
the standard 2.4 kernel RAID support.

o RAID-4

o 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 disks 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.

o If one drive fails, the parity information can be used to
reconstruct all data. If two drives fail, all data is lost.

o 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.

o RAID-5

o 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.

o 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.

o Both read and write performance usually increase, but it's hard to
predict how much.

2.3.1. 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 survive a disk crash. 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.

2.4. 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 fstab file.

A nice 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 the
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, except for when the
array is reconstructing (eg. after a new disk is inserted into a
degraded array). When 2.4 comes out this is an issue that will most
likely get addressed fairly quickly, but until then, you should
stress-test the system yourself until you are either satisfied with
the stability or conclude that you won't be swapping on RAID.

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.

3. Hardware issues

This section will mention some of the hardware concerns involved when
running software RAID.

3.1. 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.

o 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.

o 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.

o Performance: I'm not going to write thoroughly about IDE
performance here. The really short story is:

o IDE drives are fast (12 MB/s and beyond)

o IDE has more CPU overhead than SCSI (but who cares?)

o Only use one IDE drive per IDE bus, slave disks spoil performance

o 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, I'd say an IDE disk array could be a really
nice solution if one can live with the relatively low (around 8
probably) disks one can attach to a typical system (unless of course,
you have a lot of PCI slots for those IDE controllers).

IDE has major cabling problems though 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).

3.2. Hot Swap

This has been a hot topic on the linux-kernel list for some time.
Although hot swapping of drives is supported to some extent, it is
still not something one can do easily.

3.2.1. 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. The current IDE driver can't do that. If the new disk is
100% identical to the old one (wrt. geometry etc.), it may work even
without re-scanning the bus, but really, you're walking the bleeding
edge here.

3.2.2. 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.

3.2.3. Hot-swapping with SCA

With SCA, it should be possible to hot-plug devices. However, I don't
have the hardware to try this out, and I haven't heard from anyone
who's tried, so I can't really give any recipe on how to do this.

If you want to play with this, you should know about SCSI and RAID
internals anyway. So I'm not going to write something here that I
can't verify works, instead I can give a few clues:

o Grep for remove-single-device in linux/drivers/scsi/scsi.c

o Take a look at raidhotremove and raidhotadd

Not all SCSI drivers support appending and removing devices. In the
2.2 series of the kernel, at least the Adaptec 2940 and Symbios
NCR53c8xx drivers seem to support this, others may and may not. I'd
appreciate if anyone has additional facts here...

4. RAID setup

4.1. General setup

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

o A kernel. Preferably a stable 2.2.X kernel, or the latest 2.0.X.
(If 2.4 is out when you read this, go for that one instead)

o The RAID patches. There usually is a patch available for the
recent kernels. (If you found a 2.4 kernel, the patches are already
in and you can forget about them)

o The RAID tools.

o Patience, Pizza, and your favorite caffeinated beverage.

All this software can be found at ftp://ftp.fi.kernel.org/pub/linux
The RAID tools and patches are in the daemons/raid/alpha subdirectory.
The kernels are found in the kernel subdirectory.

Patch the kernel, configure it to include RAID support for the level
you want to use. Compile it and install it.

Then unpack, configure, compile and install the RAID tools.

Ok, so far so good. If you reboot now, you should have a file called
/proc/mdstat. Remember it, that file is your friend. See what it
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.

Now, let's go mode-specific.

4.2. 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.

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 fstab and so on.

4.3. 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.

4.4. 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
chunk-size 4
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.

4.5. 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.

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.

4.6. 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 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

or

raidstart /dev/md0

commands.

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

4.7. 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.

4.8. 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''.

4.8.1. 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.

4.8.2. 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, reads can be done in a parallel RAID-0 like manner.

4.8.3. 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 is the size of the
parity blocks. If one byte is written to a RAID-4 array, then chunk-
size bytes will be read from the N-1 disks, the parity information
will be calculated, and chunk-size bytes written to the parity disk.

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.

4.8.4. RAID-5

On RAID-5 the chunk-size has exactly the same meaning as in RAID-4.

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.

4.9. 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.

4.10. Autodetection

Autodetection allows the RAID devices to be automatically recognized
by the kernel at boot-time, right after the ordinary partition
detection is done.

This requires several things:

1. You need autodetection support in the kernel. Check this

2. You must have created the RAID devices using persistent-superblock

3. 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 raidstop /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.

You may want to look in your init-scripts for any raidstart/raidstop
commands. These are often found in the standard RedHat init scripts.
They are used for old-style RAID, and has no use in new-style RAID
with autodetection. Just remove the lines, and everything will be just
fine.

4.11. Booting on RAID

There are several ways to set up a system that mounts it's root
filesystem on a RAID device. At the moment, only the graphical
install of RedHat Linux 6.1 allows direct installation to a RAID
device. So most likely you're in for a little tweaking if you want
this, but it is indeed possible.

The latest official lilo distribution (Version 21) doesn't handle RAID
devices, and thus the kernel cannot be loaded at boot-time from a RAID
device. If you use this version, your /boot filesystem will have to
reside on a non-RAID device. A way to ensure that your system boots
no matter what is, to create similar /boot partitions on all drives in
your RAID, that way the BIOS can always load data from eg. the first
drive available. This requires that you do not boot with a failed
disk in your system.

With redhat 6.1 a patch to lilo 21 has become available that can
handle /boot on RAID-1. Note that it doesn't work for any other level,
RAID-1 (mirroring) is the only supported RAID level. This patch
(lilo.raid1) can be found in
dist/redhat-6.1/SRPMS/SRPMS/lilo-0.21-10.src.rpm on any redhat mirror.
The patched version of LILO will accept boot=/dev/md0 in lilo.conf and
will make each disk in the mirror bootable.

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.

4.12. 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. Because none of the current distributions (that I
know of at least) support installing on a RAID device, the methods
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.

4.12.1. 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.

o First, install a normal system on your extra disk.

o 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.

o 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.

o Just to make sure everything's fine, try rebooting the system to
see if the new RAID comes up on boot. It should.

o Put a filesystem on the new array (using mke2fs), and mount it
under /mnt/newroot

o 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

o You should modify the /mnt/newroot/etc/fstab file to use the
correct device (the /dev/md? root device) for the root filesystem.

o 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.

o 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.

o 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.

4.12.2. Method 2

This method requires that you use a raidtools/patch that includes the
failed-disk directive. This will be the tools/patch for all kernels
from 2.2.10 and later.

You can only use this method on RAID levels 1 and above. 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.

o 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!

o 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.

o Now, set up the RAID with your current root-device as the failed-
disk in the 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.

o Try rebooting and see if the RAID comes up as it should

o Copy the system files, and reconfigure the system to use the RAID
as root-device, as described in the previous section.

o When your system successfully boots from the RAID, you can modify
the raidtab file to include the previously failed-disk as a normal
raid-disk. Now, raidhotadd the disk to your RAID.

o You should now have a system that can boot from a non-degraded
RAID.

4.13. 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.

4.13.1. 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 --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.

4.14. 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.

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.

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.

Most of the ``error reports'' we see on linux-kernel, are from people
who somehow failed to use the right RAID-patch with the right version
of the raidtools. Make sure that if you're running 0.90 RAID, you're
using the raidtools for it

5. Testing

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's attached to with it, rendering all drives on that bus
inaccessible. I've never heard of that happening though. 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.

5.1. Simulating a drive failure

If you want to simulate a drive failure, then 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 ?

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.

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.

5.2. 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. Reconstruction

If you've read the rest of this HOWTO, you should already have a
pretty good idea about what reconstruction of a degraded RAID
involves. I'll summarize:

o Power down the system

o Replace the failed disk

o Power up the system once again.

o Use raidhotadd /dev/mdX /dev/sdX to re-insert the disk in the array

o Have coffee while you watch the automatic reconstruction running

And that's it.

Well, it usually is, unless you're unlucky and you 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, loosing two or more disks can be fatal.

The following 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 this will only work on kernels 2.2.10 and later.

6.1. Recovery from a multiple disk failure

The scenario is:

o A controller dies and takes two disks offline at the same time,

o All disks on one scsi bus can no longer be reached if a disk dies,

o 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.

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 won't work.

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.

7. Performance

This section contains a number of benchmarks from a real-world system
using software RAID.

Benchmarks are done with the bonnie program, and at all times on files
twice- or more the size of the physical RAM in the machine.

The benchmarks here only measures input and output bandwidth on one
large single file. This is a nice thing to know, if it's maximum I/O
throughput for large reads/writes one is interested in. However, such
numbers tell us little about what the performance would be if the
array was used for a news spool, a web-server, etc. etc. Always keep
in mind, that benchmarks numbers are the result of running a
``synthetic'' program. Few real-world programs do what bonnie does,
and although these I/O numbers are nice to look at, they are not
ultimate real-world-appliance performance indicators. Not even close.

For now, I only have results from my own machine. The setup is:

o Dual Pentium Pro 150 MHz

o 256 MB RAM (60 MHz EDO)

o Three IBM UltraStar 9ES 4.5 GB, SCSI U2W

o Adaptec 2940U2W

o One IBM UltraStar 9ES 4.5 GB, SCSI UW

o Adaptec 2940 UW

o Kernel 2.2.7 with RAID patches

The three U2W disks hang off the U2W controller, and the UW disk off
the UW controller.

It seems to be impossible to push much more than 30 MB/s thru the SCSI
busses on this system, using RAID or not. My guess is, that because
the system is fairly old, the memory bandwidth sucks, and thus limits
what can be sent thru the SCSI controllers.

7.1. RAID-0

Read is Sequential block input, and Write is Sequential block output.
File size was 1GB in all tests. The tests where done in single-user
mode. The SCSI driver was configured not to use tagged command
queuing.

| | | | |
|Chunk size | Block size | Read KB/s | Write KB/s |
| | | | |
|4k | 1k | 19712 | 18035 |
|4k | 4k | 34048 | 27061 |
|8k | 1k | 19301 | 18091 |
|8k | 4k | 33920 | 27118 |
|16k | 1k | 19330 | 18179 |
|16k | 2k | 28161 | 23682 |
|16k | 4k | 33990 | 27229 |
|32k | 1k | 19251 | 18194 |
|32k | 4k | 34071 | 26976 |

From this it seems that the RAID chunk-size doesn't make that much of
a difference. However, the ext2fs block-size should be as large as
possible, which is 4KB (eg. the page size) on IA-32.

7.2. RAID-0 with TCQ

This time, the SCSI driver was configured to use tagged command
queuing, with a queue depth of 8. Otherwise, everything's the same as
before.

| | | | |
|Chunk size | Block size | Read KB/s | Write KB/s |
| | | | |
|32k | 4k | 33617 | 27215 |

No more tests where done. TCQ seemed to slightly increase write
performance, but there really wasn't much of a difference at all.

7.3. RAID-5

The array was configured to run in RAID-5 mode, and similar tests
where done.

| | | | |
|Chunk size | Block size | Read KB/s | Write KB/s |
| | | | |
|8k | 1k | 11090 | 6874 |
|8k | 4k | 13474 | 12229 |
|32k | 1k | 11442 | 8291 |
|32k | 2k | 16089 | 10926 |
|32k | 4k | 18724 | 12627 |

Now, both the chunk-size and the block-size seems to actually make a
difference.

7.4. RAID-10

RAID-10 is ``mirrored stripes'', or, a RAID-1 array of two RAID-0
arrays. The chunk-size is the chunk sizes of both the RAID-1 array and
the two RAID-0 arrays. I did not do test where those chunk-sizes
differ, although that should be a perfectly valid setup.

| | | | |
|Chunk size | Block size | Read KB/s | Write KB/s |
| | | | |
|32k | 1k | 13753 | 11580 |
|32k | 4k | 23432 | 22249 |

No more tests where done. The file size was 900MB, because the four
partitions involved where 500 MB each, which doesn't give room for a
1G file in this setup (RAID-1 on two 1000MB arrays).

8. Credits

The following people contributed to the creation of this
documentation:

o Ingo Molnar

o Jim Warren

o Louis Mandelstam

o Allan Noah

o Yasunori Taniike

o Martin Bene

o Bennett Todd

o The Linux-RAID mailing list people

o The ones I forgot, sorry :)

Please submit corrections, suggestions etc. to the author. It's the
only way this HOWTO can improve.