This week someone asked me about an advanced setting to optimize vMotion for VPLEX configurations. This person referred to the vSphere 5.5 Performance Best Practices paper and more explicitly the following section:
Add the VMX option (extension.converttonew = “FALSE”) to virtual machine’s .vmx files. This option optimizes the opening of virtual disks during virtual machine power-on and thereby reduces switch-over time during vMotion. While this option can also be used in other situations, it is particularly helpful on VPLEX Metro deployments.
I had personally never heard of this advanced setting and I did some searches both internally and externally and couldn’t find any references other than in the vSphere 5.5 Performance paper. Strange, as you could expect with a generic recommendation like the above that it would be mentioned at least in 1 or 2 other spots. I reached out to one of the vMotion engineers and after going back and forth I figured out what the setting is for and when it should be used.
During testing with VPLEX and VMs using dozens of VMDKs in a “high latency” situation it could take longer than expected before the switchover between hosts had happened. First of all, when I say “high latency” we are talking about close to the max tolerated for VPLEX which is around 10ms RTT. When “extension.converttonew” is used the amount of IO needed during the switchover is limited, and when each IO takes 10ms you can imagine that has a direct impact on the time it takes to switchover. Of course these enhancements where also tested in scenarios where there wasn’t high latency, or a low number of disks were used, and in those cases the benefits of the enhancements were negligible and the operation overhead of configuring this setting did not weigh up against the benefits.
So to be clear, this setting should only be used in scenarios where high latency and a high number of virtual disks results in a long switchover time during migrations of VMs between hosts in a vMSC/VPLEX configuration. I hope that helps.