
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]gleblanc@cu-portland.edu>> 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:gleblanc@cu-portland.edu
  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
