Article ID: 126075, created on Jul 3, 2015, last review on Jun 17, 2016

  • Applies to:
  • Virtuozzo 6.0


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.

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