Re: raid6 grow "hangs" on 2.6.27 with mounted FS

[roma1390 <roma1390@gmail.com>]

Neil Brown wrote:
 > On Saturday March 21, roma1390@gmail.com wrote:
 > I can almost reproduce this.
 > However when I run it, the "mdadm --grow" prints     mdadm: Need to 
backup 384K of critical section..
 > (as expected) and then hangs.
 >
 > If I interrupt it and proceed, then the umount hangs.
 >
 > Is this the case for you?

Yes, mdadm hangs by itself, If i start mdadm in background and wait some 
time for reconstrucion, umount still hangs. I didn't touch mdadm, and 
mdadm likes to stop...

 > The umount hangs because there is something important that mdadm needs
 > to do which it didn't do because it was interrupted.  During the
 > 'critical section', mdadm causes all writes to the start of the device
 > to be blocked.  The umount tries to write the filesystem superblock
 > and hangs.
 > You can test if this is the problem by running the command
 >
 >  cat /sys/block/md2/md/suspend_hi > /sys/block/md2/md/suspend_lo

First try was:

   mdadm --grow /dev/md2 --raid-devices=5 &
   sleep 3
   cat /sys/block/md2/md/suspend_hi
   # output: 768
   cat /sys/block/md2/md/suspend_lo
   # output: 0
   cat /sys/block/md2/md/suspend_hi > /sys/block/md2/md/suspend_lo
   sleep 20
   umount /dev/md2

And this works! mdadm unhangs, and umount doesn't block any more.

 > That should allow the 'umount' to complete.
 >
 > This will only happen on very small arrays that take less than a
 > couple of seconds for the reshape to complete.  I have a patch for
 > mdadm which makes it more robust in this situation.  It will be in
 > future releases.
 >
 > Does this explain what is happening to you?

Yes, thanks. May be if I retest this situation with same kernel and 
limited recovery bandwith, then may be i can't hit same problem again?

 > Thanks,
 > NeilBrown

Thanks.