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

[Thanos Makatos]

(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?

-- 
Thanos Makatos