[dm-devel] dm-cache: blocks don't get cached on 3.18.21-17.el6.x86_64

[Joe Thornber]

On Mon, Mar 14, 2016 at 09:54:06AM +0000, Thanos Makatos wrote:
> (I've already reported this issue to centos and centos-devel. and
> waited long enough but didn't get any reply.)
> 
> I'm evaluating dm-cache on CentOS 6 kernels  3.18.21-17.el6.x86_64 (Xen 4) and
> 2.6.32-573.7.1.el6.x86_64 (KVM). The test I do is a simple sequential
> read using dd(1). read_promote_adjustment and sequential_threshold
> have been set to 1 and 0, respectively. For the 2.6.32 kernel, all
> seems to be working fine, "#usedcacheblocks" correctly reflects the
> number of cached blocks based on what I'm reading with dd(1),
> performance is pretty much native SSD performance. However, the same
> test on the 3.18.21-17.el6.x86_64 kernel results in "#usedcacheblocks"
> being stuck to "2" and no performance improvement is observed.
> 
> Any ideas what could be wrong? How can I further debug this?

There may not be anything wrong, dm-cache is very cagey these days
about promoting blocks to the ssd without evidence that they're
hotspots.  Hitting blocks once with a dd is not enough.

- Joe