Re: raid6 rebuild not starting

[NeilBrown <neilb@suse.de>]

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On Sun, 11 Dec 2011 09:03:14 +0200 Anssi Hannula <anssi.hannula@iki.fi> wrote:

> Hi!
> 
> After I rebooted during a raid6 rebuild, the rebuild didn't start again.
> Instead, there is a flood of "RAID conf printout"s that seemingly happen
> on array activity.
> 
> All the devices show up properly in --detail and two devices are marked
> as "spare rebuilding", and I can access the contents of the array just
> fine, but the rebuild doesn't actually start. Is this a bug or am I
> missing something? :)
> 
> I was initially on 2.6.38.8, but also tried 3.1.4 which seems to have
> the same issue. mdadm is 3.1.5.
> 
> I'm not using start_ro and writing to the array doesn't trigger a
> rebuild either.
> 
> Attached are --examine outputs before assembly, kernel log output on
> assembly, /proc/mdstat and --detail after assembly (on 3.1.4).
> 

Thank you for the very detailed problem report.

Unfortunately it is a complete mystery to me what is happening.

The repeated "RAID conf printout" messages are almost certainly coming from
the end of raid5_remove_disk.
It is being called from remove_and_add_spares for each of the two devices
that are being rebuilt.  raid5_remove_disk declines to remove them because it
can keep rebuilding them.

remove_and_add_spares then counts them and notes there are 2.
md_check_recovery notes that this is > 0, so it should create a thread to run
md_do_sync.

md_do_sync should then print out a message like
  md: recovery of RAID array md0

but it doesn't.  So something went wrong.
There are three reasons that md_do_sync might not print a message:

1/ MD_RECOVERY_DONE is set.  As only md_do_sync ever sets it, that is
    unlikely, and in any case md_check_recovery clears it.
2/ mddev->ro != 0.  It is only ever set to 0, 1, or 2.  If it is 1 or 2
   then we would be able to see that in /proc/mdstat as a "(readonly)"
   status.  But we don't.
3/ MD_RECOVERY_INTR is set. Again, md_check_recovery clears this.  It does
   get set if kthread_should_stop() returns 'true', but that should only
   happen if kthread_stop() was called.  That is only called by
   md_unregister_thread and I cannot see any way that could be call.

So.  No idea.

Are you compiling these kernels yourself?
If so, could you:
 - put a printk in the top of md_do_sync to report the values of
   mddev->recovery and mddev->ro
 - print a message whenever md_unregister_thread is called
 - in md_check_recovery, in the 
		if (mddev->ro) {
			/* Only thing we do on a ro array is remove
			 * failed devices.
			 */
			mdk_rdev_t *rdev;

  in statement, print the value of mddev->ro.

Then see which of those printk's fire, and what they tell us.

NeilBrown


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