You can adjust the maximum number of outstanding disk requests with the Disk.SchedNumReqOutstanding parameter in the vSphere Client. When two or more virtual machines are accessing the same LUN, this parameter controls the number of outstanding requests that each virtual machine can issue to the LUN. Adjusting the limit can help equalize disk access between virtual machines.

This limit does not apply when only one virtual machine is active on a LUN. In that case, the bandwidth is limited by the queue depth of the storage adapter.

1

In the vSphere Client, select the host in the inventory panel.

2

Click the Configuration tab and click Advanced Settings under Software.

3

Click Disk in the left panel and scroll down to Disk.SchedNumReqOutstanding.

4

Change the parameter value to the number of your choice and click OK.

This change can impact disk bandwidth scheduling, but experiments have shown improvements for disk-intensive workloads.

If you adjust this value in the VMkernel, you might also want to adjust the queue depth in your storage adapter.