[dm-devel] dm-cache coherence issue

[Johannes Bauer]

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?

I do think I followed a tutorial that I found on the net regarding this.
Scary that such a crucial fact is missing there. The fact that dirty
pages are reported as zero just gives the impression that everything is
coherent, when in fact it's not.

Regardless, I find this and your explanation extremely interesting and
want to thank you for clearing this up. Very fascinating topic indeed.

Best regards,
Johannes