vSAN high LLOG consumption leading to LogCongestion

Products

• VMware vSAN

Issue/Introduction

This KB article addresses an LSOM Log Congestion issue affecting vSAN versions 7.0 U1 – 7.0 U3e and 8.0 GA – 8.0c. The issue occurs when "unmapFairness" is enabled, and "GuestUnmap" is currently or was previously enabled at any point after the initial cluster deployment.

Symptoms:

Running vSAN version 7.0 U1-7.0 U3e or  vSAN  8.0 GA – 8.0c with “GuestUnmap” & “unmapFairness” enabled and one or all of the following symptoms are present in the cluster: 

• Significant performance degradation on the vSAN cluster. 
• VM operations are impacted due to cluster performance in a degraded state. 
• One or more disk group(s) experiencing LogCongestion. 
• One or more disk group (s) experiencing high levels of LLOG consumption. 

Run the following commands to verify the current vSAN UNMAP settings. 

vSAN GuestUnmap: 

esxcfg-advcfg -g /VSAN/GuestUnmap 

Value of GuestUnmap is 1 -> a value of ‘1’ means it is enabled while ‘0’ indicates it is disabled. 
 

vSAN unmapFairness: 

esxcfg-advcfg -g /LSOM/unmapFairness 

Value of unmapFairness is 1 -> a value of ‘1’ means it is enabled which is the default configuration after vSAN 7.0 U1. 

 

Environment

VMware vSAN 7.x

VMware vSAN 8.x

Resolution

• If  ESXi is on 7.x , then update to vSAN/ESXi 7.0 U3f or higher which contains the code fix for this issue
• If  ESXi is on 8.x , then update to vSAN/ESXi 8.0 U1 or higher which contains the code fix for this issue

Workaround:

If an upgrade is not possible at this time, follow the below steps to disable unmap.

1. To disable unmap, SSH into each host in the vSAN cluster and run the following commands:
   
   
   1. esxcfg-advcfg -s 0 /VSAN/GuestUnmap
   2. esxcfg-advcfg -s 0 /LSOM/unmapFairness
      
      
2. Run the following to persist the change:
   
   
   1. /sbin/autobackup.sh
      
      
3. Place the host into maintenance mode with ensure accessibility
   
   
4. Reboot the host to make the new setting active

Note: GuestUnmap is the same as the 'GuestTrimUnmap' setting for those using automation to change their configurations