On Fri Aug 28, 2015 at 04:22:09am -0500, David C. Rankin wrote:

All,

   I had a disc-controller failure on a server running several raid1
arrays. The disks are fine, but I have had the root partition come up
in degraded mode. What is the best way to tell mdraid to resync the
disks? Here are the symptoms:

# cat /proc/mdstat
Personalities : [raid1]
md1 : active raid1 sdb7[1]
       52396032 blocks super 1.2 [2/1] [_U]

md3 : active raid1 sdb6[1] sda6[0]
       1047552 blocks super 1.2 [2/2] [UU]

md2 : active raid1 sda8[0] sdb8[1]
       922944192 blocks super 1.2 [2/2] [UU]
       bitmap: 0/7 pages [0KB], 65536KB chunk

md0 : active raid1 sda5[0] sdb5[1]
       204608 blocks super 1.2 [2/2] [UU]

unused devices: <none>

<- snip ->

Reading, it looks like one approach is the boot the install media and
then zero the superblock on /dev/sda7 and then reboot. Will that force
a rebuild, or do I need to fail and remove the disk first? I was thinking:

# mdadm --zero-superblock /dev/sda7

should set it up for a rebuild without more. Is this a sane approach?

No need to over-complicate things. The only issue you have looks to be
that sda7 has not come up as part of md1, so just add it back in:
    mdadm /dev/md1 -a /dev/sda7

You probably want to check dmesg, etc. to see why it didn't get added in
at all in the first place (I'd have expected it to be at least in as a
spare).

Cheers,
    Robin
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