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Xenvbd IO performance issues and new release

Vector Support


I have been chasing down a storage performance issue for recently built Windows VMs in our XS7.1CU2 environment for quite some time now. Citrix support has been involved but has been hung up on various excuses such as irrelevant security hotfixes not being installed in the pool. We have a 7 host cluster with Dell Equallogic iSCSI storage backend. After much troubleshooting, I've finally narrowed down the issue.


Windows Server VMs (2012R2, 2016, and 2019) running the xenvbd storage driver have a 3x penalty in random read IO performance as reported by Crystal DiskMark. This mainifests as slow database read performance with a production workload. Downgrading to driver version resolves the issue. Also, upgrading to version, released today, resolves the issue.


Has anyone else experienced similar issues with xenvbd


I would consider upgrading to the latest tools package with xenvbd as the solution, except that this version drops support for quiesced snapshots. Is any insight available as to whether that feature will be reintroduced in a future 9.x xenvbd release? This is important functionality to those of us using a snapshot-based backup solution for our workloads.

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I haven't noticed, but that doesn't mean a performance issue doesn't exist. The releases of the Citrix Management Agent

are very vague about what if fixed. Not too impressed with them removing the quiesced feature either. Snapshot based

backups should still work, but if the backups relied on a quiesced clean state it could be problematic during a restore.

I personally use agent based backups within the VM so I will at least be unaffected.





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Quiesced snapshots are gone for good and are removed from the rest of the hypervisor in Citrix Hypervisor 8.1.


The functionality relied on undocumented and unsupported behaviour of the Windows Virtual Shadow Service which

  1. don't work with UEFI boot (because the disk image format changes) - that is the hypervisor disk modifications don't work on UEFI. VSS in Windows does of course work, just not the way the hypervisor abuses it.
  2. requires making binary level modifications to the disk image after the snapshot is taken to allow the disks to be used later, but these modifications are based on the undocumented behaviour and are therefore considered to be unsupportable and in the worst case dangerous to data integrity.
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Hi Mark,


Thanks for sharing!


Wow, this does not sound optimistic at all... :-( I wonder what other hypervisors make different to still allow application-aware snapshots, if that's really such a problem? And application-aware backups, ideally agentless, is, what it needs to have a nicely integrated backup solution.


BfN, Konrad

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