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

Broadcom recommends upgrading to the latest version for improved performance.
Use one of the following Workaround 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

Please contact Broadcom support for further investigation and assistance.

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.