Linux-RAID FAQ

Gregory Leblanc

             gleblanc (at) cu-portland.edu

  Revision History
  Revision v0.0.9 9 October 2000 Revised by: gml
  Updates to the location of the patches, and a couple of other things
  which I can't remember.
  Revision v0.0.8 6 September 2000 Revised by: gml
  The info/welcome message on vger.kernel.org has a pointer to this FAQ.
  New section on recovery, and fixed a few markup things.

  This is a FAQ for the Linux-RAID mailing list, hosted on
  vger.kernel.org. vger.rutgers.edu is gone, so don't bother looking for
  it. It's intended as a supplement to the existing Linux-RAID HOWTO, to
  cover questions that keep occurring on the mailing list. PLEASE read
  this document before your post to the list.
    _________________________________________________________________

  1. [1]General

       1.1. [2]Where can I find archives for the linux-raid mailing
               list?

       1.2. [3]Where can I find the latest version of this FAQ?
       1.3. [4]What sorts of things does this list cover?

  2. [5]Kernel

       2.1. [6]I'm running [insert your linux distribution here]. Do I
               need to patch my kernel to make RAID work?

       2.2. [7]How can I tell if I need to patch my kernel?
       2.3. [8]Where can I get the latest RAID patches for my kernel?
       2.4. [9]How do I apply the patch to a kernel that I just
               downloaded from ftp.kernel.org?

       2.5. [10]What kind of drives can I use RAID with? Do only SCSI or
               IDE drives work? Do I need different patches for
               different kinds of drives?

  3. [11]RAIDtools

       3.1. [12]Why are the RAIDtools at
               [13]http://people.redhat.com/mingo/raid-patches/ labeled
               dangerous, and if they're dangerous, should I use them?

       3.2. [14]Are there any tools other than the dangerous ones
               available?

  4. [15]Disk Failures and Recovery

       4.1. [16]How can I tell if one of the disks in my RAID array has
               failed?

       4.2. [17]So my RAID set is missing a disk, what do I do now?
       4.3. [18]dmesg shows "md: serializing resync, md4 has overlapping
               physical units with md5". What does this mean?

1. General

  1.1. Where can I find archives for the linux-raid mailing list?

  My favorite archives are at
  [19]http://www.geocrawler.com/lists/3/Linux/57/0/.

  Other archives are available at
  [20]http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2

  Another archive site is
  [21]http://www.mail-archive.com/linux-raid@vger.rutgers.edu/

  1.2. Where can I find the latest version of this FAQ?

  The latest version of this FAQ will be available from the LDP website
  at [22]http://www.LinuxDoc.org/FAQ/. As soon as I get my server at
  home fixed I'll make it available there as well.

  1.3. What sorts of things does this list cover?

  Well, obviously this list covers RAID in relation to Linux. Most of
  the discussions are related to the raid code that's been built into
  the Linux kernel. There are also a few discussions on getting hardware
  based RAID controllers working using Linux as the operating system.
  Any and all of these discussions are valid for this list.

2. Kernel

  2.1. I'm running [insert your linux distribution here]. Do I need to
  patch my kernel to make RAID work?

  Well, the short answer is, it depends. Some distributions are using
  the RAID 0.90 patches, while others leave the kernel with the older md
  code. Unfortunately, I don't have a list of which distributions have
  which kernels. If you'd like to maintain such a list, please email me
  <<[23][email protected]>> as well as the linux-raid mailing
  list.

  If you download a 2.2.x kernel from ftp.kernel.org, then you will need
  to patch your kernel.

  2.2. How can I tell if I need to patch my kernel?

  That depends on which kernel series you're using. If you're using the
  2.4.x kernels, then you've already got the latest RAID code that's
  available. If you're running 2.2.x, see the following instructions on
  how to find out.

  The easiest way is to check what's in /proc/mdstat. Here's a sample
  from a 2.2.x kernel, with the RAID patches applied.


[gleblanc@grego1 gleblanc]$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [translucent]
read_ahead not set
unused devices: <none>

  If the contents of /proc/mdstat looks like the above, then you don't
  need to patch your kernel.

  The "Personalities" line in your kernel may not look exactly like the
  above, if you have RAID compiled as modules. Most distributions will
  have RAID compiled as modules to save space on the boot diskette. If
  you're not using any RAID sets, then you will probably see a blank
  space at the end of the "Personalities" line, don't worry, that just
  means that the RAID modules aren't loaded yet.

  Here's a sample from a 2.2.x kernel, without the RAID patches applied.
[root@serek ~]# cat /proc/mdstat
Personalities : [1 linear] [2 raid0]
read_ahead not set
md0 : inactive
md1 : inactive
md2 : inactive
md3 : inactive


  If your /proc/mdstat looks like this one, then you need to patch your
  kernel.

  2.3. Where can I get the latest RAID patches for my kernel?

  The patches for the 2.2.x kernels up to, and including, 2.2.13 are
  available from [24]ftp.kernel.org. Use the kernel patch that most
  closely matches your kernel revision. For example, the 2.2.11 patch
  can also be used on 2.2.12 and 2.2.13.

  The patches for 2.2.14 and later kernels are at
  [25]http://people.redhat.com/mingo/raid-patches/. Use the right patch
  for your kernel, these patches haven't worked on other kernel
  revisions yet. Please use something like wget/curl/lftp to retrieve
  this patch, as it's easier on the server than using a client like
  Netscape. Downloading patches with Lynx has been unsuccessful for me;
  wget may be the easiest way.

    Note: These patches should also be available from
    [26]ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches
    / I could not find them on my local mirror, but please check yours
    before using the main kernel.org site. You can find a list of the
    local mirrors at [27]http://www.kernel.org/mirrors/.

  2.4. How do I apply the patch to a kernel that I just downloaded from
  ftp.kernel.org?

  First, unpack the kernel into some directory, generally people use
  /usr/src/linux. Change to this directory, and type patch -p1 <
  /path/to/raid-version.patch.

  On my RedHat 6.2 system, I decompressed the 2.2.16 kernel into
  /usr/src/linux-2.2.16. From /usr/src/linux-2.2.16, I type in patch -p1
  < /home/gleblanc/raid-2.2.16-A0. Then I rebuild the kernel using make
  menuconfig and related builds.

  2.5. What kind of drives can I use RAID with? Do only SCSI or IDE
  drives work? Do I need different patches for different kinds of
  drives?

  Software RAID works with any block device in the Linux kernel. This
  includes IDE and SCSI drives, as well as most harware RAID
  controllers. There are no different patches for IDE drives vs. SCSI
  drives.

3. RAIDtools

  3.1. Why are the RAIDtools at
  [28]http://people.redhat.com/mingo/raid-patches/ labeled dangerous,
  and if they're dangerous, should I use them?

  The tools are labeled dangerous because the RAID code isn't part of
  the "stable" Linux kernel.

  The tools found at the above URL are the latest and greatest. You
  should use these tools with the kernel patches from the same location.

  3.2. Are there any tools other than the dangerous ones available?

  No, the dangerous tools available from
  [29]http://people.redhat.com/mingo/raid-patches/ are the most current
  tools to use. Everyone using RAID with the patches at the above
  location should be using these dangerous tools.

4. Disk Failures and Recovery

  4.1. How can I tell if one of the disks in my RAID array has failed?

  A couple of things should indicate when a disk has failed. There
  should be quite a few messages in /var/log/messages indicating errors
  accessing that device, which should be a good indication that
  something is wrong.

  You should also notice that your /proc/mdstat looks different. Here's
  a snip from a good /proc/mdstat


[gleblanc@grego1 gleblanc]$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [translucent]
read_ahead not set
md0 : active raid1 sdb5[0] sda5[1] 32000 blocks [2/2] [UU]
unused devices: <none>

  And here's one from a /proc/mdstat where one of the RAID sets has a
  missing disk.


[gleblanc@grego1 gleblanc]$ cat /proc/mdstat
Personalities : [linear] [raid0] [raid1] [raid5] [translucent]
read_ahead not set
md0 : active raid1 sdb5[0] sda5[1] 32000 blocks [2/1] [U_]
unused devices: <none>

  I don't know if /proc/mdstat will reflect the status of a HOT SPARE.
  If you have set one up, you should be watching /var/log/messages for
  any disk failures. I'd like to get some logs of a disk failure, and
  /proc/mdstat from a system with a hot spare.

  4.2. So my RAID set is missing a disk, what do I do now?

  RAID generally doesn't mark a disk as bad unless it is, so you
  probably need a new disk. Most disks have a 3 year warranty, but some
  good SCSI hard drives may have a 5 year warranty. See if you can get
  the manufacturer to replace the failed disk for you.

  When you get the new disk, power down the system, and install it, then
  partition the drive so that it has partitions the size of your missing
  RAID partitions. After you're finished partitioning the disk, use the
  command raidhotadd to put the new disk into the array and begin
  reconstruction. See [30]Chapter 6 of the [31]Software RAID HOWTO for
  more information.

  4.3. dmesg shows "md: serializing resync, md4 has overlapping physical
  units with md5". What does this mean?

  In that message "physical units" refers to disks, and not to blocks on
  the disks. Since there is more than 1 RAID array that needs resyncing
  on a disk, the RAID code is going to sync md4 first, and md5 second,
  to avoid excessive seeks (also called thrashing), which would
  drastically slow the resync process.

References

  1. Linux-RAID-FAQ.html#AEN24
  2. Linux-RAID-FAQ.html#AEN27
  3. Linux-RAID-FAQ.html#AEN37
  4. Linux-RAID-FAQ.html#AEN43
  5. Linux-RAID-FAQ.html#AEN47
  6. Linux-RAID-FAQ.html#AEN50
  7. Linux-RAID-FAQ.html#AEN58
  8. Linux-RAID-FAQ.html#AEN74
  9. Linux-RAID-FAQ.html#AEN86
 10. Linux-RAID-FAQ.html#AEN100
 11. Linux-RAID-FAQ.html#AEN104
 12. Linux-RAID-FAQ.html#AEN107
 13. http://people.redhat.com/mingo/raid-patches/
 14. Linux-RAID-FAQ.html#AEN118
 15. Linux-RAID-FAQ.html#AEN127
 16. Linux-RAID-FAQ.html#AEN130
 17. Linux-RAID-FAQ.html#AEN146
 18. Linux-RAID-FAQ.html#AEN155
 19. http://www.geocrawler.com/lists/3/Linux/57/0/
 20. http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2
 21. http://www.mail-archive.com/linux-raid@vger.rutgers.edu/
 22. http://www.LinuxDoc.org/FAQ/
 23. mailto:[email protected]
 24. ftp://ftp.kernel.org/pub/linux/daemons/raid/alpha/
 25. http://people.redhat.com/mingo/raid-patches/
 26. ftp://ftp.kernel.org/pub/linux/kernel/people/mingo/raid-patches/
 27. http://www.kernel.org/mirrors/
 28. http://people.redhat.com/mingo/raid-patches/
 29. http://people.redhat.com/mingo/raid-patches/
 30. http://www.LinuxDoc.org/HOWTO/Software-RAID-HOWTO-6.html
 31. http://www.LinuxDoc.org/HOWTO/Software-RAID-HOWTO.html

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