Re: dom0 suddenly blocking on all access to md device

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hello,

On Fri, Feb 26, 2021 at 10:39:27PM +0000, Andy Smith wrote:
> just lately I've been sporadically having issues where dom0 blocks
> or severely slows down on all access to the particular md device
> that hosts all domU block devices.

This just happened again on the same server as the previous
occurrence on 26th February.

Again I was able to get things going again and avoid having to
destroy all guests and hard reset the host by doing a destroy on a
guest that was seen to be trying to do the most block IO (through
xentop).

This means that this particular host is still on Debian buster
kernel 4.19.0-13-amd64 and credit2 scheduler. As it's been only a
few days this might present itself again on this host quite soon. Is
there any useful information I can provide when it does?

Unfortunately there were no logs of interest neither in the dom0
syslog, guest syslog nor dom0 serial console. After the heavy IO
guest was destroyed dom0 syslog did give:

Mar  2 00:14:08 clockwork kernel: [6732447.973298] xen-blkback: Scheduled work 
from previous purge is still busy, cannot purge list

…but I assume that is just a result of IO springing back to life.

Thanks,
Andy