Linux-RAID FAQ

Gregory Leblanc

Revision History                                                             
Revision v0.0.12          2003-03-05              Revised by: gml            
Fleshed out questions that cover using mdadm, small formatting changes       
Revision v0.0.11          2003-01-08              Revised by: gml            
Updated Archive Locations, information on when to patch, added a note about  
old patches being missing, removed question about the raidtools being        
dangerous (since they don't appear to be so labeled any longer).             
Revision v0.0.10          24 April 2001           Revised by: gml            
Added a new section and question about benchmarking.                         


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.

Linux RAID FAQ Copyright

This documentation was developed for the Linux Documentation Project by
Gregory Leblanc.

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Table of Contents
1. Linux RAID FAQ

1. Linux RAID FAQ

1. General
    1.1.1. Where can I find archives for the linux-raid mailing list?
    1.1.2. Where can I find the latest version of this FAQ?
    1.1.3. What sorts of things does this list cover?
   
   
2. Kernel
    1.2.1. I'm running [insert your linux distribution here]. Do I need to
        patch my kernel to make RAID work?
    1.2.2. How can I tell if I need to patch my kernel?
    1.2.3. Where can I get the latest RAID patches for my kernel?
    1.2.4. How do I apply the patch to a kernel that I just downloaded from
        ftp.kernel.org?
    1.2.5. 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?
   
   
3. RAID tools
    1.3.1. What tools are available for dealing with my Linux Software RAID
        arrays?
   
   
4. Disk Failures and Recovery
    1.4.1. How can I tell if one of the disks in my RAID array has failed?
    1.4.2. So my RAID set is missing a disk, what do I do now?
    1.4.3. dmesg shows "md: serializing resync, md4 has overlapping physical
        units with md5" (where md4 and md5 are two of your software RAID
        devices). What does this mean?
   
   
5. Benchmarking
    1.5.1. How should I benchmark my RAID devices? Are there any tools that
        work particularly well?
   
   

1. General

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

The only archives left seem to be available at [http://marc.theaimsgroup.com
/?l=linux-raid&r=1&w=2] http://marc.theaimsgroup.com/?l=linux-raid&r=1&w=2

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

The latest version of this FAQ will be available from the LDP website at
[http://www.tldp.org/FAQ/] http://www.tldp.org/FAQ/.

1.1.3. 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

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

At this point, most major distributions are shipping with a 2.4 based kernel,
which already includes the necessary patches. If your distribution is still
using a 2.2.x kernel, upgrade!

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

1.2.2. 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.

1.2.3. 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 [ftp://ftp.kernel.org/pub/linux/daemons/raid/alpha/] 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.

Important These patches are no longer available from this location! I haven't
          been able to find the new location for them, please email me if you
          know where they've gone.                                           

The patches for 2.2.14 and later kernels are at [http://people.redhat.com/
mingo/raid-patches/] http://people.redhat.com/mingo/raid-patches/. Use the
right patch for your kernel, these patches haven't worked on other kernel
revisions. 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.

1.2.4. 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.

1.2.5. 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. RAID tools

1.3.1. What tools are available for dealing with my Linux Software RAID
arrays?

There are currently two sets of tools available. Both sets work quite well,
and have essentially the same functionalty. I recommend the newer set of
tools, because they're much easier to use, but I'll mention where to get the
older tools as well.

The new set of tools is called mdadm. It doesn't have much of a homepage, but
you can download tarballs and RPMs from [http://www.cse.unsw.edu.au/~neilb/
source/mdadm/] http://www.cse.unsw.edu.au/~neilb/source/mdadm/. I suggest
that anyone who isn't already familar with the 'raidtools' package use these
(and in fact, I suggest that folks who already know the raidtools package
switch over to these).

The older set of tools is called raidtools. They're available from [http://
people.redhat.com/mingo/raidtools/] http://people.redhat.com/mingo/raidtools
/. I believe there are other locations available, since Red Hat Linux is
shipping based on a tarball numbered 1.00.3, which I can't find online. If
anybody knows where these are, please let me know.

4. Disk Failures and Recovery

1.4.1. 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.

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

Software-RAID generally doesn't mark a disk as bad unless it is, so you
probably need a new disk. Most decent quality disks have a 3 year warranty,
but some exceptional (and expensive) SCSI hard drives may have wararnties as
long as 5 years, or even longer. More and more hard drive vendors are giving
a 1 year warranty on their "consumer" drives. I suggest avoiding any drive
with a 1 year warranty if at all possible. Try to have the manufacturer
replace the failed disk if it's still under warranty.

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. Once you have the partitions set up properly, just run mdadm
--add /dev/md0 /dev/hdc1, where /dev/md0 is the RAID array you're adding the
partition to, and /dev/hdc1 is the partition that you're trying to add.
Reconstruction should start immediately.

If you would prefer to use the RAIDtools suite, you can use the command 
raidhotadd to put the new disk into the array and begin reconstruction. See 
Chapter 6 of the Software RAID HOWTO for more information.

1.4.3. dmesg shows "md: serializing resync, md4 has overlapping physical
units with md5" (where md4 and md5 are two of your software RAID devices).
What does this mean?

In that message "physical units" refers to disks, and not to blocks on the
disks. Since there is more than one RAID array that needs resyncing on one of
the disks in use for your RAID arrays, 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.

5. Benchmarking

1.5.1. How should I benchmark my RAID devices? Are there any tools that work
particularly well?

There are really a few options for benchmarking your RAID array, depending on
what you're looking to test. RAID offers the greatest speed increases when
there are multiple threads reading from the same RAID volume.

One tool specificly designed to test and show off these performance gains is 
tiobench. It uses multiple read and write threads on the disk, and has some
pretty good reporting.

Another good tool to use is bonnie++. It seems to be more targeted at
benchmarking single drives that at RAID, but still provides useful
information.

One tool NOT to use is hdparm. It does not give useful performance numbers
for any drives that I've heard about, and has been known to give some
incredibly off-the-wall numbers as well. If you want to do real benchmarking,
use one of the tools listed above.
