Capacity-drive space congestion and write latency observed with Compression-Only enabled.

Products

• VMware vSAN

Issue/Introduction

Symptoms:

High VM disk write latency on IO sensitive VMs.

Required Conditions:

All of the below must be true for this issue

• vSAN 7.0U1 and higher or vSAN 8.0 OSA, releases prior to 8.0 U2.
• Compression-Only is enabled.
• A vSAN disk group 70% full or higher.

Environment

VMware vSAN 7.0.x

VMware vSAN 8.0.x

Cause

The issue is triggered when a vSAN disk group exceeds 70% full and the pending deletes exceed 10% of the Write Buffer size (600GB).

These pending deletes for the disk group can happen simply by normal background vSAN rebalancing (which is a very likely event as the disk group becomes more full), by deleting VMs or vDisks, or by svMotion out of the vSAN cluster.

A temporary throttle of incoming I/O might be introduced in order to avoid running out of space during large delete operations resulting in a temporary drop in performance (latency spike) to be experienced by Virtual Machines.

Resolution

Upgrade the vCenter and ESXi to 8.0U2 and later versions.

Additional Information

Impact/Risks:

Latency sensitive Virtual Machines (DB VMs) might report errors within Guest OS. Linux based Virtual Machines might remount it's disks into Read Only.