Le Mon, 16 Feb 2009 13:34:50 +0100,
Rémi PALANCHER [off-list ref] a écrit :

Hi there,

I've been a happy and successful mdadm user for a while now, but
unfortunately, I met a problem recently.

I use mdadm to manage a RAID5 array with 5 disks and 1 spare. The
devices in the array are sda5, sdb1, sdc1, sdd1, sde1 and sdf1. Before
things start to go wrong, sdb1 device was the spare and the other
devices were active in the array. Here is this configuration as shown
by the logs:

[85054.372171] RAID5 conf printout:
[85054.372175]  --- rd:5 wd:4
[85054.372177]  disk 0, o:1, dev:sdc1
[85054.372179]  disk 1, o:1, dev:sde1
[85054.372181]  disk 2, o:1, dev:sda5
[85054.372183]  disk 3, o:1, dev:sdd1
[85054.372185]  disk 4, o:0, dev:sdf1

Last thursday, a problem occured on sdf1 and mdadm started recovering
the array with sdb1 as expected. But sdb1 have had a problem too
during the recovery process, but the array continued to run properly
with the last good 4 devices. I can attach logs if needed. Actually,
sdb1 was defective, and I removed it physically from the server.

But here is the output when I try to reassemble the array with the
remaining devices:
# mdadm --assemble /dev/md0 --force \
--verbose /dev/sda5 /dev/sdb1 /dev/sdc1 /dev/sdd1 /dev/sde1
mdadm: looking for devices for /dev/md0
mdadm: /dev/sda5 is identified as a member of /dev/md0, slot 5.
mdadm: /dev/sdb1 is identified as a member of /dev/md0, slot 0.
mdadm: /dev/sdc1 is identified as a member of /dev/md0, slot 3.
mdadm: /dev/sdd1 is identified as a member of /dev/md0, slot 1.
mdadm: /dev/sde1 is identified as a member of /dev/md0, slot -1.
mdadm: added /dev/sdd1 to /dev/md0 as 1
mdadm: no uptodate device for slot 2 of /dev/md0
mdadm: added /dev/sdc1 to /dev/md0 as 3
mdadm: no uptodate device for slot 4 of /dev/md0
mdadm: added /dev/sda5 to /dev/md0 as 5
mdadm: added /dev/sde1 to /dev/md0 as -1
mdadm: added /dev/sdb1 to /dev/md0 as 0
mdadm: /dev/md0 assembled from 3 drives and 2 spares - not enough to
start the array.

I can understand why sde1 (previously sdf1) cannot take an active
slot, but why does sda5 wanna take slot5 instead of slot2, as it was
previously?

Is there any way to force it?

FYI, I finally found a (ugly) way to force sda5 to take slot2 by doing
this:

# mdadm --run /dev/md0
(An error was shown here, but devices appeared in /sys regardless)
# cat /sys/block/md0/md/dev-sda5/slot
5
# echo 2 > /sys/block/md0/md/dev-sda5/slot
# mount -o ro /dev/md0

It's quite ugly and probably not the best way to do it, but fortunately
it worked!

-- 
Rémi PALANCHER             Ingénieur associé
Administrateur systèmes et réseaux Grid'5000
INRIA Saclay                      Bâtiment H
tel : 01 74 85 42 45            Bureau : 015 
mob : 06 31 99 95 17                       

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