software-raid.howto.txt

create raid tool at linux

  The Software-RAID HOWTO
  Jakob OEstergaard ([e-mail])
  v. 0.90.2 - Alpha, 27th february 1999

  This HOWTO describes how to use Software RAID under Linux. You must be
  using the RAID patches available from ftp://ftp.fi.ker-
  nel.org/pub/linux/daemons/raid/alpha. The HOWTO can be found at
  http://ostenfeld.dk/~jakob/Software-RAID.HOWTO/.
  ______________________________________________________________________

  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. RAID setup

     3.1 General setup
     3.2 Linear mode
     3.3 RAID-0
     3.4 RAID-1
     3.5 RAID-4
     3.6 RAID-5
     3.7 The Persistent Superblock
     3.8 Chunk sizes
        3.8.1 RAID-0
        3.8.2 RAID-1
        3.8.3 RAID-4
        3.8.4 RAID-5
     3.9 Options for mke2fs
     3.10 Autodetection
     3.11 Booting on RAID
     3.12 Pitfalls

  4. Credits



  ______________________________________________________________________

  1.  Introduction

  This howto is written by Jakob OEstergaard based on a large number of
  emails between the author and Ingo Molnar ([e-mail])
  -- one of the RAID developers --, the linux-raid mailing list (linux-
  [e-mail]) and various other people.

  The reason this HOWTO was written even though a Software-RAID HOWTO
  allready exists is, that the old HOWTO describes the old-style
  Software RAID found in the stock 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.

  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 loose 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.



  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. Both
  can be found at ftp://ftp.fi.kernel.org/pub/linux/daemons/raid/alpha.
  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 loose 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  Read performance will usually scale close to to N*P, while write
     performance is the same as on one device, or perhaps even less.
     Reads can be done in parallel, but when writing, the CPU must
     transfer N times as much data to the disks as it usually would
     (remember, N identical copies of all data must be sent to the
     disks).

  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 writte 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.

  However, swap on RAID-{1,4,5} is NOT supported. You can set it up, but
  it will crash. The reason is, that the RAID layer sometimes allocates
  memory before doing a write. This leads to a deadlock, since the
  kernel will have to allocate memory before it can swap, and swap
  before it can allocate memory.

  It's sad but true, at least for now.



  3.  RAID setup


  3.1.  General setup

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

  o  A kernel.  Get 2.0.36 or a recent 2.2.x kernel.

  o  The RAID patches.  There usually is a patch available for the
     recent kernels.

  o  The RAID tools.

  o  Patience, Pizza, and your favourite caffeinated beverage.


  All this software can be found at ftp://ftp.fi.kernel.org/pub/linux