Re: dm-cache coherence issue

[Joe Thornber <thornber@redhat.com>]

On Mon, Jun 26, 2017 at 10:36:23PM +0200, Johannes Bauer wrote:
> On 26.06.2017 21:56, Mike Snitzer wrote:
> 
> >> Interesting, I did *not* change to writethrough. However, there
> >> shouldn't have been any I/O on the device (it was not accessed by
> >> anything after I switched to the cleaner policy).
> [...]
> >> Anyways, I'll try to replicate my scenario again because I'm actually
> >> quite sure that I did everything correctly (I did it a few times).
> > 
> > Except you didn't first switch to writethrough -- which is _not_
> > correct.
> 
> Absolutely, very good to know. So even without any I/O being request,
> dm-cache is allowed to "hold back" pages as long as the dm-cache device
> is in writeback mode? Would this also explain why the "dmsetup wait"
> hung indefinitely?

Some things to try to see if it makes a difference:

- unmount the cache before checksumming it so we know there's no IO from
  the page cache going in.

- deactivate the cache before checksumming the origin.

- Stop using encryption on top of cache and see if that makes a
  difference.

- Use 'dmsetup wait' properly, as Mike mentioned.  See the following code
  from dmtest:

    https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/cache_utils.rb#L28
    https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/device-mapper/event_tracker.rb

- use md5sum rather than your checksum program.  Humour me.



- Joe