Increased Latency for I/O Requests Slows Virtual Machine Performance

If the ESXi host generates more commands to a LUN than the LUN queue depth permits, the excess commands are queued in VMkernel. This increases the latency, or the time taken to complete I/O requests.

Problem

The host takes longer to complete I/O requests and virtual machines display unsatisfactory performance.

Cause

The problem might be caused by an inadequate LUN queue depth. SCSI device drivers have a configurable parameter called the LUN queue depth that determines how many commands to a given LUN can be active at one time. If the host generates more commands to a LUN, the excess commands are queued in the VMkernel.

Solution

1. If the sum of active commands from all virtual machines consistently exceeds the LUN depth, increase the queue depth.
   The procedure that you use to increase the queue depth depends on the type of storage adapter the host uses.
2. When multiple virtual machines are active on a LUN, change the Disk.SchedNumReqOutstanding (DSNRO) parameter, so that it matches the queue depth value.