High Read/Write latency with RDM Disks (MSCS) on 5.1

vmware When you upgrade to 5.1, the default path selection policy for active/active arrays is now Round Robin (RR). In 5.0, It would be Fixed PSP. The reason I wanted to stress on this is that MSCS cluster does not support using RR. And when you use it you will see significant performance degradation. The VM seems to be running fine but slow, event when creating a folder, you may have to wait. One other good indicators is the significant increase in the backup times after an upgrade.

You can check out performance graphs and counters. In my case, there was no dropped commands but upto 5000ms latency on read and writes, and when I run esxtop, it was clear that there was something wrong as DAVG was around 50 and KAVG was around 5000. You can also change default PSP if you need to.

vSphere 5.1 changes default multipathing policy for EMC Symmetrix devices to Round Robin (2034797)

If you want to change the defaults:

Changing a LUN to use a different Path Selection Policy (PSP) (1036189)

The screenshot below does show normal values for the same device after changing the PSP for RDM disk to Most recently used (MRU).

KAVG

DAVG/cmd: the average response time in milliseconds per command being sent to the device, threshold 25

KAVG/cmd: the amount of time the command spends in the VMkernel

Good luck

Bulent Tolu

Bulent Tolu

Sr. Systems Engineer at VMware
Bulent is an IT professional with Master's in MIS and 10-years of experience in broad range of Information Technologies. He is exposed to engineering/architecting, implementation/integration, and administration of various high-available IT systems and infrastructure. He has a passion to continually research, test and evaluate new technologies and follow industry best practices to secure and optimize IT systems. Currently, he lives in Istanbul and works as a Sr. Systems Engineer at VMware.
Bulent Tolu

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