Large I/O block size operations show high latency in Windows 2008 in ESX 4.x and ESXi 4.x / 5.x (2036863)
This issue can also be reproduced in a Linux virtual machine with the dd command and issuing commands with a large block size (i.e. - 1M).
Note: Reducing the Disk.DiskMaxIOSize advanced setting in the ESX/ESXi host does not improve the latency results because the Guest operating system is the one issuing large I/O block size. For more information, see Tuning ESX/ESXi for better storage performance by modifying the maximum I/O block size (1003469).
The Windows registry may be altered to issue smaller I/O block sizes, resulting in lower latency. However, this is merely a false positive. For more information, see:
- Large I/Os Are Split into 64KB Units When Using LSILogic Driver (9645697)
- The Guest Operating System Storage Considerations section in the Performance Best Practices for VMware vSphere 5.0.
While one may think that issuing a virtual machine migration or virtual machine deployment can be considered a large block operation, in reality the vmkernel issues I/Os block size of 32k. So, this is significantly smaller than what the guest operating system issues, resulting in no high latency for this operation.