[dm-devel] dm-cache coherence issue

[Johannes Bauer]

Hi Joe,

On 26.06.2017 17:58, Joe Thornber wrote:
> On Mon, Jun 26, 2017 at 12:33:42PM +0100, Joe Thornber wrote:
>> On Sat, Jun 24, 2017 at 03:56:54PM +0200, Johannes Bauer wrote:
>>> So I seem to have a very basic misunderstanding of what the cleaner
>>> policy/dirty pages mean. Is there a way to force the cache to flush
>>> entirely? Apparently, "dmsetup wait" and/or "sync" don't do the job.
>>
>> I'll try and reproduce your scenario and get back to you.
> 
> Here's a similar scenario that I've added to the dm test suite:
> 
> https://github.com/jthornber/device-mapper-test-suite/commit/457e889b0c4d510609c0d7464af07f2ebee20768
> 
> It goes through all the steps you need to use to decommission a cache
> with dirty blocks.  Namely:
> 
> - switch to writethrough mode (so new io can't create new dirty blocks)
> - switch to the cleaner policy
> - wait for clean
> - remove the cache device before accessing the origin directly

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).

On the advice of Zdenek Kabelac, who messaged me off-list, I therefore
have indeed been convinced that using dm-cache "by hand" maybe is too
dangerous (i.e., I had not accounted for "repairing" of a cache and am
really still unsure what LVM does that) and that lvmcache is a better
choice -- even though Ubuntu supports it really crappily for root devices:
https://bugs.launchpad.net/ubuntu/+source/lvm2/+bug/1423796 a shame,
it's been like this for over two years.

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).

Thanks for you help,
Johannes