Detecting, querying and testing

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m (Simulating a drive failure)
(mdadmisation and some updates)
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learn more about /proc/mdstat interpretation.
 
learn more about /proc/mdstat interpretation.
  
==Querying the arrays status==
+
==Querying the array status==
  
You can always take a look at /proc/mdstat. It won't hurt. Let's learn
+
You can always take a look at the array status by doing '''cat /proc/mdstat'''
how to read the file. For example,
+
It won't hurt. Let's learn how to read the file. For example,
  
 
                 Personalities : [raid1]
 
                 Personalities : [raid1]
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                 md5 : active raid1 sdb5[1] sda5[0]
 
                 md5 : active raid1 sdb5[1] sda5[0]
 
                       4200896 blocks [2/2] [UU]
 
                       4200896 blocks [2/2] [UU]
 
+
 
 
                 md6 : active raid1 sdb6[1] sda6[0]
 
                 md6 : active raid1 sdb6[1] sda6[0]
 
                       2104384 blocks [2/2] [UU]
 
                       2104384 blocks [2/2] [UU]
 
+
 
 
                 md7 : active raid1 sdb7[1] sda7[0]
 
                 md7 : active raid1 sdb7[1] sda7[0]
 
                       2104384 blocks [2/2] [UU]
 
                       2104384 blocks [2/2] [UU]
 
+
 
 
                 md2 : active raid1 sdc7[1] sdd8[2] sde5[0]
 
                 md2 : active raid1 sdc7[1] sdd8[2] sde5[0]
 
                       1052160 blocks [2/2] [UU]
 
                       1052160 blocks [2/2] [UU]
 
+
 
 
                 unused devices: none
 
                 unused devices: none
  
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nothing.
 
nothing.
  
Finally, remember that you can always use raidtools or mdadm to check
+
Finally, remember that you can always use mdadm to check
 
the arrays out.
 
the arrays out.
  
 
           mdadm --detail /dev/mdx
 
           mdadm --detail /dev/mdx
          lsraid -a /dev/mdx
 
  
  
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know what will happen if a drive dies. It may electrically take the
 
know what will happen if a drive dies. It may electrically take the
 
bus it is attached to with it, rendering all drives on that bus
 
bus it is attached to with it, rendering all drives on that bus
inaccessible. I have never heard of that happening though, but it is
+
inaccessible. The drive may also just report a read/write fault
entirely possible. The drive may also just report a read/write fault
+
to the SCSI/IDE/SATA layer, which, if done properly, in turn makes the RAID layer handle this
to the SCSI/IDE layer, which in turn makes the RAID layer handle this
+
 
situation gracefully.  This is fortunately the way things often go.
 
situation gracefully.  This is fortunately the way things often go.
  
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Look in the syslog, and look at /proc/mdstat to see how the RAID is
 
Look in the syslog, and look at /proc/mdstat to see how the RAID is
doing. Did it work?
+
doing. Did it work? Did you get an email from the mdadm monitor?
  
 
Faulty disks should appear marked with an (F) if you look at
 
Faulty disks should appear marked with an (F) if you look at
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When you've re-connected the disk again (with the power off, of
 
When you've re-connected the disk again (with the power off, of
 
course, remember), you can add the "new" device to the RAID again,
 
course, remember), you can add the "new" device to the RAID again,
with the raidhotadd command.
+
with the ''mdadm --add''' command.
  
  
 
===Force-fail by software===
 
===Force-fail by software===
  
Newer versions of raidtools come with a raidsetfaulty command.  By
+
You can just simulate a drive failure without unplugging things.
using raidsetfaulty you can just simulate a drive failure without
+
unplugging things off.
+
 
+
 
Just running the command
 
Just running the command
  
       raidsetfaulty /dev/md1 /dev/sdc2
+
       mdadm --manage --set-faulty /dev/md1 /dev/sdc2
  
 
+
should be enough to fail the disk /dev/sdc2 of the array /dev/md1.
should be enough to fail the disk /dev/sdc2 of the array /dev/md1. If
+
you are using mdadm, just type
+
 
+
      mdadm --manage --set-faulty /dev/md1 /dev/sdc2
+
  
  
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spare disk available, reconstruction should have started.
 
spare disk available, reconstruction should have started.
  
Another fresh utility in newest raidtools is lsraid. Try with
+
Another useful command at this point is:
 
+
      lsraid -a /dev/md1
+
 
+
 
+
users of mdadm can run the command
+
  
 
       mdadm --detail /dev/md1
 
       mdadm --detail /dev/md1
  
 
+
Enjoy the view.
and enjoy the view.
+
  
 
Now you've seen how it goes when a device fails. Let's fix things up.
 
Now you've seen how it goes when a device fails. Let's fix things up.
  
 
First, we will remove the failed disk from the array. Run the command
 
First, we will remove the failed disk from the array. Run the command
 
 
      raidhotremove /dev/md1 /dev/sdc2
 
 
 
users of mdadm can run the command
 
  
 
       mdadm /dev/md1 -r /dev/sdc2
 
       mdadm /dev/md1 -r /dev/sdc2
  
 
+
Note that mdadm cannot pull a disk out of a running array.
Note that raidhotremove cannot pull a disk out of a running array.
+
For obvious reasons, only faulty disks can be hot-removed from an
For obvious reasons, only crashed disks are to be hotremoved from an
+
array (even stopping and unmounting the device won't help - if you ever want
array (running raidstop and unmounting the device won't help).
+
to remove a 'good' disk, you have to tell the array to put it into the
 +
'failed' state as above).
  
 
Now we have a /dev/md1 which has just lost a device. This could be a
 
Now we have a /dev/md1 which has just lost a device. This could be a
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So the trip ends when we send /dev/sdc2 back home.
 
So the trip ends when we send /dev/sdc2 back home.
 
      raidhotadd /dev/md1 /dev/sdc2
 
 
 
As usual, you can use mdadm instead of raidtools. This should be the
 
command
 
  
 
       mdadm /dev/md1 -a /dev/sdc2
 
       mdadm /dev/md1 -a /dev/sdc2
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As the prodigal son returns to the array, we'll see it becoming an
 
As the prodigal son returns to the array, we'll see it becoming an
 
active member of /dev/md1 if necessary. If not, it will be marked as
 
active member of /dev/md1 if necessary. If not, it will be marked as
an spare disk.  That's management made easy.
+
a spare disk.  That's management made easy.
  
 
==Simulating data corruption==
 
==Simulating data corruption==
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     mdadm --monitor --mail=root@localhost --delay=1800 /dev/md2
 
     mdadm --monitor --mail=root@localhost --delay=1800 /dev/md2
 
  
 
should release a mdadm daemon to monitor /dev/md2.  The delay parame-
 
should release a mdadm daemon to monitor /dev/md2.  The delay parame-
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happens, for example, if the mdadm daemon stops?  In order to overcome
 
happens, for example, if the mdadm daemon stops?  In order to overcome
 
this problem, one should look towards "real" monitoring solutions.
 
this problem, one should look towards "real" monitoring solutions.
There is a number of free software, open source, and commercial
+
There are a number of free software, open source, and even commercial
 
solutions available which can be used for Software RAID monitoring on
 
solutions available which can be used for Software RAID monitoring on
 
Linux. A search on FreshMeat should return a good number of matches.
 
Linux. A search on FreshMeat should return a good number of matches.

Revision as of 11:10, 14 November 2007

Contents

Detecting, querying and testing

This section is about life with a software RAID system, that's communicating with the arrays and tinkertoying them.

Note that when it comes to md devices manipulation, you should always remember that you are working with entire filesystems. So, although there could be some redundancy to keep your files alive, you must proceed with caution.


Detecting a drive failure

Firstly: mdadm has an excellent 'monitor' mode which will send an email when a problem is detected in any array (more about that later).

Of course he standard log and stat files will record more details about a drive failure.

It's always a must for /var/log/messages to fill screens with tons of error messages, no matter what happened. But, when it's about a disk crash, huge lots of kernel errors are reported. Some nasty examples, for the masochists,

    kernel: scsi0 channel 0 : resetting for second half of retries.
    kernel: SCSI bus is being reset for host 0 channel 0.
    kernel: scsi0: Sending Bus Device Reset CCB #2666 to Target 0
    kernel: scsi0: Bus Device Reset CCB #2666 to Target 0 Completed
    kernel: scsi : aborting command due to timeout : pid 2649, scsi0, channel 0, id 0, lun 0 Write (6) 18 33 11 24 00
    kernel: scsi0: Aborting CCB #2669 to Target 0
    kernel: SCSI host 0 channel 0 reset (pid 2644) timed out - trying harder
    kernel: SCSI bus is being reset for host 0 channel 0.
    kernel: scsi0: CCB #2669 to Target 0 Aborted
    kernel: scsi0: Resetting BusLogic BT-958 due to Target 0
    kernel: scsi0: *** BusLogic BT-958 Initialized Successfully ***

Most often, disk failures look like these,

    kernel: sidisk I/O error: dev 08:01, sector 1590410
    kernel: SCSI disk error : host 0 channel 0 id 0 lun 0 return code = 28000002

or these

    kernel: hde: read_intr: error=0x10 { SectorIdNotFound }, CHS=31563/14/35, sector=0
    kernel: hde: read_intr: status=0x59 { DriveReady SeekComplete DataRequest Error }


And, as expected, the classic /proc/mdstat look will also reveal problems,

    Personalities : [linear] [raid0] [raid1] [translucent]
    read_ahead not set
    md7 : active raid1 sdc9[0] sdd5[8] 32000 blocks [2/1] [U_]


Later on this section we will learn how to monitor RAID with mdadm so we can receive alert reports about disk failures. Now it's time to learn more about /proc/mdstat interpretation.

Querying the array status

You can always take a look at the array status by doing cat /proc/mdstat It won't hurt. Let's learn how to read the file. For example,

               Personalities : [raid1]
               read_ahead 1024 sectors
               md5 : active raid1 sdb5[1] sda5[0]
                     4200896 blocks [2/2] [UU]
 
               md6 : active raid1 sdb6[1] sda6[0]
                     2104384 blocks [2/2] [UU]
 
               md7 : active raid1 sdb7[1] sda7[0]
                     2104384 blocks [2/2] [UU]
 
               md2 : active raid1 sdc7[1] sdd8[2] sde5[0]
                     1052160 blocks [2/2] [UU]
 
               unused devices: none


To identify the spare devices, first look for the [#/#] value on a line. The first number is the number of a complete raid device as defined. Lets say it is "n". The raid role numbers [#] following each device indicate its role, or function, within the raid set. Any device with "n" or higher are spare disks. 0,1,..,n-1 are for the working array.

Also, if you have a failure, the failed device will be marked with (F) after the [#]. The spare that replaces this device will be the device with the lowest role number n or higher that is not marked (F). Once the resync operation is complete, the device's role numbers are swapped.

The order in which the devices appear in the /proc/mdstat output means nothing.

Finally, remember that you can always use mdadm to check the arrays out.

         mdadm --detail /dev/mdx


These commands will show spare and failed disks loud and clear.

Simulating a drive failure

If you plan to use RAID to get fault-tolerance, you may also want to test your setup, to see if it really works. Now, how does one simulate a disk failure?

The short story is, that you can't, except perhaps for putting a fire axe thru the drive you want to "simulate" the fault on. You can never know what will happen if a drive dies. It may electrically take the bus it is attached to with it, rendering all drives on that bus inaccessible. The drive may also just report a read/write fault to the SCSI/IDE/SATA layer, which, if done properly, in turn makes the RAID layer handle this situation gracefully. This is fortunately the way things often go.

Remember, that you must be running RAID-{1,4,5,6,10} for your array to be able to survive a disk failure. Linear- or RAID-0 will fail completely when a device is missing.

Force-fail by hardware

If you want to simulate a drive failure, you can just plug out the drive. You should do this with the power off. If you are interested in testing whether your data can survive with a disk less than the usual number, there is no point in being a hot-plug cowboy here. Take the system down, unplug the disk, and boot it up again.

Look in the syslog, and look at /proc/mdstat to see how the RAID is doing. Did it work? Did you get an email from the mdadm monitor?

Faulty disks should appear marked with an (F) if you look at /proc/mdstat. Also, users of mdadm should see the device state as faulty.

When you've re-connected the disk again (with the power off, of course, remember), you can add the "new" device to the RAID again, with the mdadm --add' command.


Force-fail by software

You can just simulate a drive failure without unplugging things. Just running the command

     mdadm --manage --set-faulty /dev/md1 /dev/sdc2

should be enough to fail the disk /dev/sdc2 of the array /dev/md1.


Now things move up and fun appears. First, you should see something like the first line of this on your system's log. Something like the second line will appear if you have spare disks configured.

     kernel: raid1: Disk failure on sdc2, disabling device.
     kernel: md1: resyncing spare disk sdb7 to replace failed disk


Checking /proc/mdstat out will show the degraded array. If there was a spare disk available, reconstruction should have started.

Another useful command at this point is:

     mdadm --detail /dev/md1

Enjoy the view.

Now you've seen how it goes when a device fails. Let's fix things up.

First, we will remove the failed disk from the array. Run the command

     mdadm /dev/md1 -r /dev/sdc2

Note that mdadm cannot pull a disk out of a running array. For obvious reasons, only faulty disks can be hot-removed from an array (even stopping and unmounting the device won't help - if you ever want to remove a 'good' disk, you have to tell the array to put it into the 'failed' state as above).

Now we have a /dev/md1 which has just lost a device. This could be a degraded RAID or perhaps a system in the middle of a reconstruction process. We wait until recovery ends before setting things back to normal.

So the trip ends when we send /dev/sdc2 back home.

     mdadm /dev/md1 -a /dev/sdc2


As the prodigal son returns to the array, we'll see it becoming an active member of /dev/md1 if necessary. If not, it will be marked as a spare disk. That's management made easy.

Simulating data corruption

RAID (be it hardware- or software-), assumes that if a write to a disk doesn't return an error, then the write was successful. Therefore, if your disk corrupts data without returning an error, your data will become corrupted. This is of course very unlikely to happen, but it is possible, and it would result in a corrupt filesystem.

RAID cannot and is not supposed to guard against data corruption on the media. Therefore, it doesn't make any sense either, to purposely corrupt data (using dd for example) on a disk to see how the RAID system will handle that. It is most likely (unless you corrupt the RAID superblock) that the RAID layer will never find out about the corruption, but your filesystem on the RAID device will be corrupted.

This is the way things are supposed to work. RAID is not a guarantee for data integrity, it just allows you to keep your data if a disk dies (that is, with RAID levels above or equal one, of course).


Monitoring RAID arrays

You can run mdadm as a daemon by using the follow-monitor mode. If needed, that will make mdadm send email alerts to the system administrator when arrays encounter errors or fail. Also, follow mode can be used to trigger contingency commands if a disk fails, like giving a second chance to a failed disk by removing and reinserting it, so a non-fatal failure could be automatically solved.

Let's see a basic example. Running

    mdadm --monitor --mail=root@localhost --delay=1800 /dev/md2

should release a mdadm daemon to monitor /dev/md2. The delay parame- ter means that polling will be done in intervals of 1800 seconds. Finally, critical events and fatal errors should be e-mailed to the system manager. That's RAID monitoring made easy.

Finally, the --program or --alert parameters specify the program to be run whenever an event is detected.

Note that the mdadm daemon will never exit once it decides that there are arrays to monitor, so it should normally be run in the background. Remember that your are running a daemon, not a shell command.

Using mdadm to monitor a RAID array is simple and effective. However, there are fundamental problems with that kind of monitoring - what happens, for example, if the mdadm daemon stops? In order to overcome this problem, one should look towards "real" monitoring solutions. There are a number of free software, open source, and even commercial solutions available which can be used for Software RAID monitoring on Linux. A search on FreshMeat should return a good number of matches.

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