Virtual machines performance degradation was observed during Storage vMotion operation.

Products

• VMware vSAN

Issue/Introduction

Symptoms:

• Performance degradation observed during Storage vMotion operations from legacy SAN storage to the vSAN environment.

Environment

VMware vSAN 8.x

VMware vSAN 9.x

Cause

The performance bottleneck is attributed to an imbalanced Cache-to-Capacity ratio. A significant deficit in cache size relative to capacity leads to delayed data de-staging and increased I/O congestion during heavy migration workloads.

For optimal vSAN performance, the recommended Cache-to-Capacity ratio should be approximately 1:10. The current environment exceeds this ratio, preventing the cache tier from efficiently handling the sustained write buffers required during large-scale data migrations.

Resolution

• Expand Cache Tier: Increase the cache disk size to align with the 1:10 ratio to alleviate congestion and improve de-staging speeds.
• Policy Optimization: Temporarily change the VM Storage Policy to FTT=0 during the migration process to reduce the number of components placed across hosts, thereby streamlining performance.

Solution Benefits

According to the Broadcom vSAN Large Cache Support documentation, increasing the cache size beyond 600 GB (supported in vSAN 8.0+) provides:

• Write Optimization: Enhanced handling of write-intensive applications and large block random writes.
• Reduced Latency: Up to 10x improvement in latency and CPU efficiency for specific Working Set Sizes (WSS).
• Extended Burst Phases: Sustained performance for burst workloads lasting 2x–3x longer than in previous versions.
• Operational Speed: Significantly faster host reboots (up to 50%) and improved resync/rebuild durations.