Poor read performance in KVM

[Vladimir Bashkirtsev <vladimir@bashkirtsev.com>]

Hello,

Lately I was trying to get KVM to perform well on RBD. But it still 
appears elusive.

[root@alpha etc]# rados -p rbd bench 120 seq -t 8

Total time run:        16.873277
Total reads made:     302
Read size:            4194304
Bandwidth (MB/sec):    71.592

Average Latency:       0.437984
Max latency:           3.26817
Min latency:           0.015786

Fairly good performance. But when I run in KVM:

[root@mail ~]# hdparm -tT /dev/vda

/dev/vda:
  Timing cached reads:   8808 MB in  2.00 seconds = 4411.49 MB/sec
  Timing buffered disk reads:  10 MB in  6.21 seconds =   1.61 MB/sec

Not even close to what rados bench show! I even seen 900KB/sec 
performance. Such slow read performance of course affecting guests.

Any ideas where to start to look for performance boost?

Regards,
Vladimir