Re: raid5 on 2.4.21 and reconstruction problem

On Thursday August 7, arekm@pld-linux.org wrote:

Hi,

I have two raid 5 partitions - md0 (as for root fs) and md1 (for /home).

...

[root@perfo arekm]# mdadm -D /dev/md0
/dev/md0:
        Version : 00.90.00
  Creation Time : Tue May  4 09:37:53 1999
     Raid Level : raid5
     Array Size : 9960064 (9.50 GiB 10.20 GB)
    Device Size : 4980032 (4.75 GiB 5.10 GB)
   Raid Devices : 3
  Total Devices : 4
Preferred Minor : 0
    Persistence : Superblock is persistent

    Update Time : Thu Aug  7 12:48:34 2003
          State : dirty, no-errors
 Active Devices : 2
Working Devices : 0
 Failed Devices : 3 
  Spare Devices : 0

"Spare Devices : 0" is the problem.  It thinks there aren't any
spares, even though there obviously is one...
The code in the kernel for keeping these counters up-to-date is very
fragile and I'm not surprised that it occasionally gets things wrong.

Both arrays was created under some 2.2 kernel. Resync doesn't happen on 2.5.25, too.

2.5.25 is ancient!
2.6.0-test2 should be able to assemble it (it ignores those counts)
but there is some data corruption bug that I am hitting in
2.6.0-test2 that could well be raid5 related, so I'm not sure I
would recommend that.

I could probably knock up a patch to 2.4.21 in a couple of days that
corrects the counts when an array is assembled.
Alternatively if you can boot off a rescue disk, then
  mdadm -C /dev/md0 -l5 -c 32 -n 3 /dev/sda2 /dev/sdb2 missing
  mdadm /dev/md0 -a /dev/sdc2

should get you out of trouble.

NeilBrown