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We have a problem with one of our deployments of Windows Server 2012 Hyper-V with a 2 node cluster connected to a iSCSI SAN.

Our Problem

  • Our problem is that for some reason all of the VMs stops responding or responds very slowly and you can for instance not send CTRL-ALT-DEL to a VM in the Hyper-V console, or for instance start task manager when already logged in.

Symptoms (i.e. this happens, or does not happen, at the same time)

  • I we look at resource monitor on the host then we see that there is often an extensive read from a VHDX of one of the VMs (40-60 Mbyte/s) and a combined write speed to many files in \HarddiskVolume5\System Volume Information{}.
  • The combined network speed to the iSCSI SAN is about 500-600 Mbit/s. When this happens it is usually during and after a VSS ShadowCopy backup, but has also happens during hours where no backup should be running (i.e. during daytime when the backup has finished hours ago according to the log files). There is however not that extensive writes to the backup file that is created on an external hard drive and this does not seem to happen during all backups (we have manually checked a few times, but it is hard to say since this error does not seem leave any traces in event viewer).
  • We cannot find any indication that the VMs themself detect any problem and we see no increase of errors (for example storage related errors) in the eventlog inside the VMs.
  • The QNAP uses about 50% processing Power on all cores.
  • We see no dropped packets on the switch.

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