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

[Felix Zielcke]

Now that I think more about this, I think this is more wishlist then a
bug.
Though I don't have a better bug title yet in mind.
So please feel free to change this.

Am Dienstag, den 19.08.2008, 19:21 +0200 schrieb martin f krafft:
> 
> Unless you waited for /dev/sdc1 to resync, removing /dev/sdd1 will
> basically destroy the array. Not much one can do about that.

I looked now again at the FAQ, question 18 looks calculatable.
I haven't bothered to look at the code, I don't understand yet that much
C and so I need a while to get into it.
But you have the numbers `mdadm -Q --detail' prints probable anyway
already.
If one device is removed the superblock is changed, so it could print
out a warning/error that it's now broken.
`mdadm -Q --detail' is telling me `clean,degraded' with one active disk
and 3 spare.
It could say `broken'.

> Anyway, I think that's the problem: when you (re-)add a disk to an
> array, it becomes a spare until it synchronised. If you wait for the
> synchronisation between each add/remove step, you should never see
> more than 1 spare.

Ah, I assumed that a spare is always only used if one disk fails.
Would it be possible to make a `mdadm --add-this-disk-as-active-instead-of-spare' ?
or is this actually a good or bad idea?

I have now added another virtual disk /dev/sdg to the VM.
Recreated the raid10
Then I set faulty and removed sdc1 and added sdg1
Still 1 is removed and 2 spare disks and no resyncing, so I think this idea isn't that bad.

fz-vm:~# mdadm -Q --detail /dev/md0
/dev/md0:
        Version : 00.90
  Creation Time : Tue Aug 19 19:52:16 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 : 5
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Tue Aug 19 20:22:52 2008
          State : active, degraded
 Active Devices : 3
Working Devices : 5
 Failed Devices : 0
  Spare Devices : 2

         Layout : near=2, far=1
     Chunk Size : 64K

           UUID : b3d68384:06515443:89ccbef7:ff5abfb0 (local to host fz-vm)
         Events : 0.21

    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
       5       8       97        -      spare   /dev/sdg1