Enable DomClientReadCache for the smaller working dataset.

Products

• VMware vSAN
• VMware vSphere ESXi

Issue/Introduction

Symptoms:

• In the case of the small dataset ( working dataset < 5GB), the user might observe performance degradation.

• In Horizon Instant Clones post vSAN upgrade to VMware ESXi 8.0.3 build-24280767 faeces performnace degradation issue.

• However, prior to the vSAN upgrade, the same Instant Clone VMs works fine on VMware ESXi 7.0.U3 build.

• High Read latency from vSAN level impacts the slowness on VDI VMs.

Environment

VMware vSAN 8.0.2

Cause

Disabling DomClientReadCache shows a significant reduction in CPU usage and an enhancement in overall performance, particularly in scenarios involving large datasets. This is attributed to the fact that the current cache size is insufficient to yield performance benefits due to a low cache hit ratio. Consequently, the decision was made to maintain DomClientReadCache as disabled by default.

However, in situations involving small datasets, DomClientReadCache could effectively store data in the cache since working datasets are small. Despite this, we opted to disable DomClientReadCache by default to address performance concerns in general cases, resulting in a performance drawback for scenarios involving small datasets.

Resolution

Currently there is no resolution to the issue.

Workaround:

To workaround the issue, please enable the client-cache with an advanced config option:

• esxcfg-advcfg -s 1 /VSAN/DomClientReadCache
  
  
  • Reboot is not required to enable the parameter.

Note: The DomClientReadCache is disabled in VSAN version 8.0u2. This workaround will be required for VSAN versions later than 8.0u2.