software-raid.howto.txt

create raid tool at 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.



  3.2.  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
          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.

  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.



  3.3.  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.



  3.4.  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.



  3.5.  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.




  3.6.  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.



  3.7.  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
  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.




  3.8.  Chunk sizes