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6. 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.

6.1 Detecting a drive failure

No mistery here. It's enough with a quick look to the standard log and stat files to notice 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.

6.2 Querying the arrays status

You can always take a look at /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 raidtools or mdadm to check the arrays out.

        mdadm --detail /dev/mdx
        lsraid -a /dev/mdx
These commands will show spare and failed disks loud and clear.

6.3 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. I have never heard of that happening though, but it is entirely possible. The drive may also just report a read/write fault to the SCSI/IDE layer, which 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} 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?

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 raidhotadd command.

Force-fail by software

Newer versions of raidtools come with a raidsetfaulty command. By using raidsetfaulty you can just simulate a drive failure without unplugging things off.

Just running the command

    raidsetfaulty /dev/md1 /dev/sdc2
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
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 fresh utility in newest raidtools is lsraid. Try with

    lsraid -a /dev/md1
users of mdadm can run the command
    mdadm --detail /dev/md1
and 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

    raidhotremove /dev/md1 /dev/sdc2
users of mdadm can run the command
    mdadm /dev/md1 -r /dev/sdc2
Note that raidhotremove cannot pull a disk out of a running array. For obvious reasons, only crashed disks are to be hotremoved from an array (running raidstop and unmounting the device won't help).

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.

    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
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 an spare disk. That's management made easy.

6.4 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).

6.5 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 parameter 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 is a number of free software, open source, and 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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