[CentOS] 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?