On Tue, 25 Nov 2014 12:49:12 +1100 Jonathan Molyneux
[off-list ref] wrote:

Hi Everyone,

Have a strange situation that hasn't happened before.
Running Debian 7.7 with kernel version 3.2.63-2+deb7u1.
Have a raid10 that runs the server (boot's off a raid1) that after 
replacing a failed disk, just won't rebuild.

This is what it looks like without the disk (failed & removed):
md1 : active raid10 sda2[6] sdc2[4] sdb2[1]
       1952987136 blocks super 1.2 512K chunks 2 far-copies [4/3] [UUU_]
       bitmap: 8/15 pages [32KB], 65536KB chunk

Then when the disk is added:
md1 : active raid10 sdd2[5](S) sda2[6] sdc2[4] sdb2[1]
       1952987136 blocks super 1.2 512K chunks 2 far-copies [4/3] [UUU_]
       bitmap: 8/15 pages [32KB], 65536KB chunk

Nothing unusual is being spat out in dmesg.
When removing the disk:
[313434.073997] md: unbind<sdd2>
[313434.138307] md: export_rdev(sdd2)
When adding the disk:
[313468.056484] md: bind<sdd2>

This is a strange one that I haven't had before.
Any thoughts on how to kick the rebuild off without needing a reboot ?

I'm sure I've seen this bug before... and fixed it.
I don't remember the details and cannot find anything obvious in change logs.

You could try

   echo recover > /sys/block/md1/md/sync_action

Alternately, if you are re-adding a disk that had just been removed, you could

   mdadm /dev/md1 --remove /dev/sdd2
   mdadm --zero /dev/sdd2
   mdadm /dev/md1 --add /dev/sdd2

that will force a full recovery instead of just a bitmap-based recovery.
That will of course take longer than a bitmap-based recover, but seeing the
bitmap based recovery isn't starting, that could still be an improvement.

NeilBrown