Last couple of weeks I heard people either asking questions around how tot limit a VM from an IOps perspective or making comments that Storage IO Control (SIOC) allows you to limit VMs. As I pointed at least three folks to this info I figured I would share it publicly.
There is an IOps limit setting on the virtual disk as an option… This is what allows you to limit a virtual machine / virtual disk to a specific amount of IOps. Now it should be noted that when you set this limit this is handled (vSphere 5.1 and prior) by the local host scheduler, also known as SFQ. One thing to realize though is that when you set a limit on multiple virtual disks for a virtual machine is that all of these limits will be added up and that will be your threshold. In other words:
- Disk01 – 50 IOps limit
- Disk02 – 200 IOps limit
- Combined total: 250 IOps limit
- If Disk01 only uses 5 IOps then Disk02 can use 245 IOps!
There is one caveat though, “combined total” only goes for the disks which are stored on the same datastore. So if you have 4 disks and they are stored across 4 datastores then each of the individual limits apply respectively.
More details can be found in this KB article: http://kb.vmware.com/kb/1038241