Virtual machines work slow with disks on a pstorage cluster, getting PAUSED from time to time due to slow IO operations.
More than 25% of total IO requests appear unaligned for these virtual machines:
[root@pcs1 ~]# prl_perf_ctl -c aio Performance Counters View Utility v6.8.23717.1100559 Copyright 1999-2015 Parallels Holdings, Ltd. and its affiliates. VM1 - 19:08:08 devices.aio.aligned 65528102 devices.aio.bbuf 0 devices.aio.read_unaligned 147 devices.aio.write_unaligned 37694142 VM2 - 19:08:08 devices.aio.aligned 81026276 devices.aio.bbuf 0 devices.aio.read_unaligned 167 devices.aio.write_unaligned 25478222 VM3 - 19:08:08 devices.aio.aligned 142506616 devices.aio.bbuf 0 devices.aio.read_unaligned 523 devices.aio.write_unaligned 51038784
Unaligned IO leads to read-modify-write operations, which greatly reduce disk performance.
Check disk partitioning for the affected VMs according to the Aligning Disks and Partitions in Virtual Machines guide.