Bug report for rbd cache setting

Hi, I'm using ceph nautilus for openstack, I found an odd problem(or a bug?) while turn on the rbd cache: 1. run a rbd read bench rbd -c /etc/ceph/ceph.conf -k /etc/ceph/keyring2 -n client.rbd-openstack-002 bench \         --io-size 4K \         --io-threads 1 \         --io-pattern rand \         --io-type read \         --io-total 100G \         openstack-volumes/image1 bench  type read io_size 4096 io_threads 1 bytes 107374182400 pattern random   SEC       OPS   OPS/SEC   BYTES/SEC     1       180    176.75  723985.61     2       369    185.37  759263.46     3       581    194.26  795669.09     4       742    184.27  754778.64 2. run another test for write(the same host or not) rbd -n client.rbd-openstack-002 -c /etc/ceph/ceph.conf -k /etc/ceph/keyring.rbd-openstack-002 bench \     --io-size 4K \     --io-threads 1 \     --io-pattern rand \     --io-type write \     --io-total 100G \     openstack-volumes/image2 3. waiting for seconds, check the read ops: bench  type read io_size 4096 io_threads 1 bytes 107374182400 pattern random   SEC       OPS   OPS/SEC   BYTES/SEC     1       140    137.16  561794.28     2       249    125.50  514046.00     3       384    128.16  524942.96    10       432     39.83  163128.47    11       435     39.48  161700.52    12       457     28.86  118208.87    25       473      9.38  38414.24    26       483      4.30  17621.06    29       494      3.42  14005.46    30       536      5.33  21828.19    31       555      5.16  21144.12    33       558     10.77  44114.68    34       576     11.22  45938.31    36       591     12.20  49987.93    37       601      9.29  38055.28 The read op is blocked by write? I think it should not be reasonable.