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degraded RAID1: how to read /proc/mdstat
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Vieri
l33t
l33t


Joined: 18 Dec 2005
Posts: 870

PostPosted: Mon Sep 22, 2014 9:15 am    Post subject: degraded RAID1: how to read /proc/mdstat Reply with quote

Hi,

I setup 2 hard disks with 4 partitions each and all of them as RAID1.

Both of my drives were working fine until recently one failed when I checked /proc/mdstat:

Code:

# cat /proc/mdstat
Personalities : [raid10] [raid1] [raid6] [raid5] [raid4] [raid0] [linear] [multipath]
md127 : active raid1 sda1[0] sdb1[1]
      1984 blocks [2/2] [UU]

md2 : active raid1 sdb2[1] sda2[0]
      131008 blocks [2/2] [UU]

md3 : active raid1 sdb3[1] sda3[2](F)
      524224 blocks [2/1] [_U]

md4 : active raid1 sdb4[1] sda4[2](F)
      292274240 blocks [2/1] [_U]

unused devices: <none>


Let's take /dev/md4 as an example:

Code:

# mdadm --query --detail /dev/md4
/dev/md4:
        Version : 0.90
  Creation Time : Fri Jul 18 18:08:59 2014
     Raid Level : raid1
     Array Size : 292274240 (278.73 GiB 299.29 GB)
  Used Dev Size : 292274240 (278.73 GiB 299.29 GB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 4
    Persistence : Superblock is persistent

    Update Time : Mon Sep 22 11:05:59 2014
          State : clean, degraded
 Active Devices : 1
Working Devices : 1
 Failed Devices : 1
  Spare Devices : 0

           UUID : f6414992:af74e556:cb201669:f728008a
         Events : 0.771421

    Number   Major   Minor   RaidDevice State
       0       0        0        0      removed
       1       8       20        1      active sync   /dev/sdb4

       2       8        4        -      faulty   /dev/sda4


I'd like to know how to read the above outputs, expecially /proc/mdstat. I understand that sda4 is failing but what does sda4[2] mean? [2] seams to mean "device number 2". However, my system only has 2 hard disks: [0] and [1]. So why is /proc/mdstat reporting [2]?

If I take a look at /dev/md2 which is not degraded:

Code:

# mdadm --query --detail /dev/md2
/dev/md2:
        Version : 0.90
  Creation Time : Fri Jul 18 18:08:59 2014
     Raid Level : raid1
     Array Size : 131008 (127.96 MiB 134.15 MB)
  Used Dev Size : 131008 (127.96 MiB 134.15 MB)
   Raid Devices : 2
  Total Devices : 2
Preferred Minor : 2
    Persistence : Superblock is persistent

    Update Time : Fri Jul 25 09:32:58 2014
          State : clean
 Active Devices : 2
Working Devices : 2
 Failed Devices : 0
  Spare Devices : 0

           UUID : 5e089a5f:7b824c9a:cb201669:f728008a
         Events : 0.18

    Number   Major   Minor   RaidDevice State
       0       8        2        0      active sync   /dev/sda2
       1       8       18        1      active sync   /dev/sdb2


So for /dev/md2 sda2 is on device 0 and sdb2 is on device 1.

I guess that when a device/partition fails (such as sda4), the device number it had is removed (ie. [0]), marked faulty and replaced by a number greater than the number of physical devices (in this case my real devices are [0] and [1] so it assigned [2]).

Am I getting it right or am I misinterpreting the output?

Thanks,

Vieri
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eccerr0r
Watchman
Watchman


Joined: 01 Jul 2004
Posts: 9645
Location: almost Mile High in the USA

PostPosted: Wed Oct 01, 2014 10:34 pm    Post subject: Reply with quote

Only thing I can think of is that when you created the array, somehow you got a phantom device that was never really used... So device 0 is not really there and everything got shifted by one...
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