CentOS - Mar 2016 - Fwd: dm-cache not caching data on 3.18.21-17.el6.x86_64

(I've already sent this to centos-devel almost a week ago but didn't
get any reply.)

I'm evaluating dm-cache on 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