vSAN Performance is degraded when I/Os are not 4k aligned

Products

• VMware vSAN

Issue/Introduction

This KB is written to inform of this known limitation and provide resources to implement known workarounds and solutions.

Symptoms:
vSAN Performance is less than expected with possible higher latency and lower throughput coupled with I/O sizes issued that are not 4k aligned.

Environment

VMware vSAN

Cause

vSAN is designed to work best with 4k aligned I/O (4k, 8k, 16k, 32k, 64k, etc) from guests. When I/O is not aligned to a 4k size, additional I/O operations are required at the vSAN layer before the write can be completed. This adds time (latency) until these operations are completed.

You may use I/O Insight to determine if non-alignment is a factor by looking at the I/O patterns of specific VMs.

• This tool is built in as of 7.0 Update 1: Precise Performance Monitoring with vSAN IOInsight
• For versions before 7.0 Update 1 upgrade vCenter to 7.0U1 or higher

Resolution

Ensure that the guest operating system and applications are set to use 4k blocks for all I/O operations. Engage your application team/software vendor to ensure the OS/application is using 4k blocks and if not configure the I/O for 4k block size.

Impact/Risks:
Any changes to the guest operating system or application should be discussed with the relevant vendor.

Additional Information

Refer : FAQ: Support statement for 512e and 4K Native drives for VMware vSphere and vSAN