On Mon, 26 May 2014 20:07:11 +0200 Sebastian Herbszt [off-list ref] wrote:

Hello,

I am wondering how to identify a failed md array.
Lets assume the following array

/dev/md0:
        Version : 1.2
  Creation Time : Mon May 26 19:10:59 2014
     Raid Level : raid1
     Array Size : 10176 (9.94 MiB 10.42 MB)
  Used Dev Size : 10176 (9.94 MiB 10.42 MB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon May 26 19:10:59 2014
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           Name : test:0  (local to host test)
           UUID : cac8fd48:44219a96:5de7e757:4e21a3e2
         Events : 17

    Number   Major   Minor   RaidDevice State
       0     254        0        0      active sync   /dev/dm-0
       1     254        1        1      active sync   /dev/dm-1

with

/sys/block/md0/md/array_state:clean
/sys/block/md0/md/dev-dm-0/state:in_sync
/sys/block/md0/md/dev-dm-1/state:in_sync

and

disk0: 0 20480 linear 7:0 0
disk1: 0 20480 linear 7:1 0

If dm-0 gets changed to "disk0: 0 20480 error" and we read from the
array (dd if=/dev/md0 count=1 iflag=direct of=/dev/null) the broken
disk gets detected by md:

[84688.483607] md/raid1:md0: dm-0: rescheduling sector 0
[84688.483654] md/raid1:md0: redirecting sector 0 to other mirror: dm-1
[84688.483670] md: super_written gets error=-5, uptodate=0
[84688.483672] md/raid1:md0: Disk failure on dm-0, disabling device.
md/raid1:md0: Operation continuing on 1 devices.
[84688.483676] md: super_written gets error=-5, uptodate=0
[84688.494174] RAID1 conf printout:
[84688.494178]  --- wd:1 rd:2
[84688.494181]  disk 0, wo:1, o:0, dev:dm-0
[84688.494182]  disk 1, wo:0, o:1, dev:dm-1
[84688.494183] RAID1 conf printout:
[84688.494184]  --- wd:1 rd:2
[84688.494184]  disk 1, wo:0, o:1, dev:dm-1

/dev/md0:
        Version : 1.2
  Creation Time : Mon May 26 19:10:59 2014
     Raid Level : raid1
     Array Size : 10176 (9.94 MiB 10.42 MB)
  Used Dev Size : 10176 (9.94 MiB 10.42 MB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon May 26 19:27:41 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           Name : test:0  (local to host test)
           UUID : cac8fd48:44219a96:5de7e757:4e21a3e2
         Events : 20

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1     254        1        1      active sync   /dev/dm-1

       0     254        0        -      faulty   /dev/dm-0

md0 : active raid1 dm-1[1] dm-0[0](F)
      10176 blocks super 1.2 [2/1] [_U]

/sys/block/md0/md/array_state:clean
/sys/block/md0/md/dev-dm-0/state:faulty,write_error
/sys/block/md0/md/dev-dm-1/state:in_sync
/sys/block/md0/md/degraded:1

However if I also change dm-1 to "disk1: 0 20480 error" and read
again there is no visible state change:

/dev/md0:
        Version : 1.2
  Creation Time : Mon May 26 19:10:59 2014
     Raid Level : raid1
     Array Size : 10176 (9.94 MiB 10.42 MB)
  Used Dev Size : 10176 (9.94 MiB 10.42 MB)
   Raid Devices : 2
  Total Devices : 2
    Persistence : Superblock is persistent

    Update Time : Mon May 26 19:27:41 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1     254        1        1      active sync   /dev/dm-1

       0     254        0        -      faulty   /dev/dm-0

md0 : active raid1 dm-1[1] dm-0[0](F)
      10176 blocks super 1.2 [2/1] [_U]

/sys/block/md0/md/array_state:clean
/sys/block/md0/md/dev-dm-0/state:faulty,write_error
/sys/block/md0/md/dev-dm-1/state:in_sync
/sys/block/md0/md/degraded:1

On write to the array we get

[85498.660247] md: super_written gets error=-5, uptodate=0
[85498.666464] quiet_error: 268 callbacks suppressed
[85498.666470] Buffer I/O error on device md0, logical block 2528
[85498.666476] Buffer I/O error on device md0, logical block 2528
[85498.666486] Buffer I/O error on device md0, logical block 2542
[85498.666490] Buffer I/O error on device md0, logical block 2542
[85498.666496] Buffer I/O error on device md0, logical block 0
[85498.666499] Buffer I/O error on device md0, logical block 0
[85498.666508] Buffer I/O error on device md0, logical block 1
[85498.666512] Buffer I/O error on device md0, logical block 1
[85498.666518] Buffer I/O error on device md0, logical block 2543
[85498.666524] Buffer I/O error on device md0, logical block 2543
[85498.866388] md: super_written gets error=-5, uptodate=0

and the only change is

/sys/block/md0/md/dev-dm-1/state:in_sync,write_error,want_replacement

How can I identify a failed array?
array_state reports "clean", the last raid member stays "in_sync" and
the value in degraded doesn't equal raid_disks.

You know the array is "failed" when you get an IO error.

When a RAID1 array gets down to just one drive remaining, it starts acting
like it is just one drive.
How do you tell if is single plain ordinary drive is failed?  You get an IO
error.  ditto with RAID1.

NeilBrown