Bug#495580: mdadm: 4 disk raid10 with 1 active and 3 spare possible

[Neil Brown]

On Wednesday August 20, fzielcke at z-51.de wrote:
> 
> The current Debian sid kernel 2.6.26-2 does sync the disks if I add
> them.
> I tried now even the newer 2.6.27-rc3-git6 out, because it has a few MD
> changes.
> That one adds the disks only as `spare' but does not start a resync.

I cannot reproduce this.

This is surprising though:

> fz-vm:~# cat /proc/mdstat
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] 
> md0 : active raid10 sdc1[4](S) sdf1[3] sde1[2] sdd1[1]
>       16771584 blocks 64K chunks 2 near-copies [4/3] [_UUU]
>       
> unused devices: <none>

Here there is one failed device.  There is one spare but the array
isn't resyncing.

And...

> fz-vm:~# mdadm -Q --detail /dev/md0
> /dev/md0:
>         Version : 00.90
>   Creation Time : Wed Aug 20 09:03:27 2008
>      Raid Level : raid10
>      Array Size : 16771584 (15.99 GiB 17.17 GB)
>   Used Dev Size : 8385792 (8.00 GiB 8.59 GB)
>    Raid Devices : 4
>   Total Devices : 4
> Preferred Minor : 0
>     Persistence : Superblock is persistent
> 
>     Update Time : Wed Aug 20 09:07:32 2008
>           State : clean, degraded
>  Active Devices : 3
> Working Devices : 4
>  Failed Devices : 0
>   Spare Devices : 1
> 
>          Layout : near=2, far=1
>      Chunk Size : 64K
> 
>            UUID : 04e72c15:980e57a0:89ccbef7:ff5abfb0 (local to host fz-vm)
>          Events : 0.10
> 
>     Number   Major   Minor   RaidDevice State
>        0       0        0        0      removed
>        1       8       49        1      active sync   /dev/sdd1
>        2       8       65        2      active sync   /dev/sde1
>        3       8       81        3      active sync   /dev/sdf1
> 
>        4       8       33        -      spare   /dev/sdc1
> fz-vm:~# cat /proc/mdstat
> Personalities : [linear] [raid0] [raid1] [raid10] [raid6] [raid5] [raid4] [multipath] 
> md0 : active raid10 sdd1[4](S) sdc1[5](S) sdf1[3] sde1[2]
>       16771584 blocks 64K chunks 2 near-copies [4/2] [__UU]

Here there are two failed devices.  What happened?

Do the kernel logs show anything between these two "cat /proc/mdstat"s?


NeilBrown