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CPU masking support in XenServer 5.6

Citrix has just released some more information about its CPU masking technology for XenServer.  Citrix calls it Heterogeneous resource pools which require a XenServer Enterprise or Platinum license. This technology is similar to VMware’s Enhanced VMotion Compatibility (EVC).

http://community.citrix.com/display/ocb/2010/10/26/CPU+masking+support+in+XenServer+5.6

These features use the capabilities built into the CPUs, either Intel’s FlexMigration or AMD’s Extended Migration to allow the configuration of a CPU to be changed by applying a CPU mask so it appears to provide different features than it actually does. This allows pools or clusters of hosts with different CPUs (from the same vendor) to support live migrations.

This is extremely useful as even CPUs with the same model number can have some differences which could cause XenMotion / Vmotion to fail.  Newer generation servers with faster CPUs and even additional cores can be added into existing pools / clusters without any downtime.

Generally you need to start with hosts with the lowest capability CPUs and then add the newer revision ones which when added to the pool / cluster mask CPU features not available with the original CPUs.  This can be done with all VMs online as the new hosts have the masks applied before any VMs start to run on the new host, maintaining compatibility with the existing pool /cluster.

If you are adding older hosts into a pool / cluster you would need to amend the mask of existing hosts which would mean all VMs would need to be shut down for this to work as a guest VM cannot downgrade its CPU capabilities while running.

Citrix has also helpfully released a Heterogeneous CPU Pool Self-Test Kit so you can check your CPU compatibility.

http://www.citrix.com/ready/hcl

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