Re: Synchronous vs asynchonous mdadm operations

[Chris Webb <chris@arachsys.com>]

Chris Webb <chris@arachsys.com> writes:

[Re: mdadm --stop being potentially asynchronous]
> The reason for the question is that I'm seeing occasional cases of arrays which
> won't reassemble following such an operation. dmesg alleges there is an invalid
> superblock for all of the six slots which were originally part of the array.

I tracked this one down to my scripts, which were failing to adjust the
available space on the rdevs in a particularly rare case. However, I'm still
wondering about the best way to do a fail/remove combination, given
that fail appears to be asynchronous. The shell fragment I give below seems
way over the top, but I can't see any simpler route....

> I notice that some mdadm operations appear to be asynchronous. For instance,
> 
>   mdadm --fail /dev/md/shelf.51000 /dev/mapper/slot.51000.1
>   mdadm --remove /dev/md/shelf.51000 /dev/mapper/slot.51000.1
> 
> will always fail at the --remove stage with
> 
>   mdadm: hot remove failed for /dev/mapper/slot.51000.1: Device or resource busy
> 
> whereas adding a short sleep in between will make it successful.
> 
> Is there a 'standard' way to wait for this operation to complete or to
> perform both steps in one go, other than something horrible like:
> 
>   mdadm --fail /dev/md/shelf.51000 /dev/mapper/slot.51000.1
>   MD=$((`stat -c '%#T' -L /dev/md/shelf.51000`))
>   MAJOR=$((`stat -c '%#t' -L /dev/mapper/slot.51000.1`))
>   MINOR=$((`stat -c '%#T' -L /dev/mapper/slot.51000.1`))
>   for RD in /sys/block/md$MD/md/rd*; do
>     [ -f $RD/block/dev ] || continue
>     [ "`<$RD/block/dev`" = "$MAJOR:$MINOR" ] || continue
>     while [ "< $RD/state" != "faulty ]; do sleep 0.1; done
>   done
>   mdadm --remove /dev/md/shelf.51000 /dev/mapper/slot.51000.1

Cheers,

Chris.