On the Prowl for vSphere Performance Tweaks

I'm always on the prowl for small tweaks that can individually make a small difference in the overall experience of a system. In the case of virtualization, this can allow me to squeeze a little more out of my systems.

For performance tweaks, of course the de facto vSphere resource is the Performance Management Best Practices document. But, while I chew through the document when it is updated, it doesn't have as many very specific configurations as I would like. There is logic in that, as you can get in trouble by over-configuring virtual machines, host servers or the storage.

With all that being said, I have come across one configuration that is in the ‘tweak' category which has been positive to my installations. The disk.schednumreqoutstanding value specifies a limit to the maximum number of outstanding disk requests to a specific LUN. The default value is 32, and for most situations is good enough. I have set this parameter to the maximum value (256) and have been very happy with the results. This value is set for each host in the advanced settings area (see Fig. 1).

Figure 1. This advanced value allows 256 requests to go to a single LUN. (Click image to view larger version.)

I came across this somewhat obscure value from a number of posts from some of the best. EMC's Chad Sakacc's post on "VMFS – Best Practices and Counter-FUD mentions" this value, as well as VMware's Duncan Epping's post "Queuedepth and What's Next?" Even VMware KB article 1268 states that if you have more than one virtual machine per LUN, which most of us do, there can be benefits during intensive workloads.

Quantifying the value of this tweak will be up to you running a test before and after with an intensive workload. I know I liked what I saw, and it is in my standard configuration now.

Posted by Rick Vanover on 04/20/2010 at 12:47 PM