Under certain conditions, Guest OS UNMAP Operation may cause temporary slower performance after deleting and re-copying data that nearly fills the volume

Products

• VMware vSAN

Issue/Introduction

When a volume is nearly full, Broadcom has observed a condition whereby a file copy operation may encounter temporary performance degradation if copied immediately after file deletion due to guest OS UNMAP behavior.

On a VM running Windows guest OS with TRIM/UNMAP enabled, where the volume is nearly full, if a large file deletion is immediately followed by a file copy operation, that copy operation may observe slowness due to the way Windows issues TRIM.

Environment

VMware vSphere ESX 9.0.x , 8.0.x

Resolution

Use one of the following methods.

1. Use a larger volume ideally 2x the size of the largest file being deleted.
2. Introduce a short interval (eg.. 30 seconds) after a large delete before starting the file copy.
3. Disable UNMAP in Windows guest OS using the command

fsutil behavior set DisableDeleteNotify 1

Additional Information

This issue is specific to environments where UNMAP is enabled in the guest OS and large file operations are performed in rapid succession on small volumes.