There is also one point that we need to measure writing performance, not reading one. For example, my ssd-backed cache will fit perfectly for a Ceph journal but will give poor numbers after read performance measurement:

/dev/sdb:
 Timing buffered disk reads: 432 MB in  3.00 seconds = 143.79 MB/sec

/dev/mapper/cache0:
 Timing buffered disk reads: 514 MB in  3.00 seconds = 171.32 MB/sec