Tuesday, 19 February 2013

Citrix PVS OfflineDB Support 5.6 - 6.0 - 6.1

There can be some confusion at as to what we can expect from OfflineDB support in Citrix Provisioning Services. By design the OfflineDB option will allow the farm to continue to function even if the PVS servers lose connectivity to the SQL server.

To explain the mechanics a little; with the OfflineDB option enabled the StreamProcess.exe will capture a snapshot of the SQL DB on start-up and hold it in the PVS servers' memory. This cache will be updated as farm changes happen. If the SQL server suddenly times out and we find it unreachable we swap to using this cached DB from memory to continue site operations:

OfflineDB enabless the following while connection to the Site SQL server is lost:
  • Existing Targets will continue to function
  • Targets who are already configured in the Site can be booted/re-booted.
However, the following will not longer be available to you:
  • AutoAdd target devices
  • vDisk updates
  • vDisk creation
  • Active Directory password changes
  • Stream Process startup
  • Image Update service
  • Management functions; PowerShell, MCLI, SoapServer and the Console
  • If you restart the PVS server it will NOT come back on-line as the cache needs to be built from SQL each time the stream process is started.
Tied to this topic is the issue of SQL setup as the two go hand in hand. Citrix officially test and document SQL in a mirrored environment. It's very difficult to get an official statement on SQL clustering and the best I have managed to get is that while SQL clustering has not been tested by engineering there is no reason why it should cause an issue as we use one connection string to the cluster and therefore should be unaware of the complexity of the SQL configuration.

We have seen some issues though where failover on the SQL cluster happens in the middle of some sql executions from PVS and hung threads have been detected. While these will time-out and be retried we do see evidence of this taking some time and causing hanging behaviour in the consol. This is due to be cleaned up in Hotfix 018  for PVS 6.1 rumour has it.

Its worth noting that some of the functionality we rely on when failing back on this HA design has had a number of bugs resolved in HotFix 016 for PVS found here  too.

Enabling Offline Database Support (NOT enabled by default!)
To enable the Offline Database Support option
  • In the Console tree, right-click on the Farm, then select Properties. The Farm Properties dialog appears.
  • On the Options tab, check the checkbox next to Offline Database Support.
  • Restart Stream services.

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