This KB addresses a LSOM Log Congestion issue from vSAN 7.0 U1 - 7.0 U3e versions when “unmapFairness” is enabled and “GuestUnmap” is or has been enabled at some point after the cluster was initially built.

Symptoms:

Running vSAN version 7.0 U1-7.0 U3e 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. 

Update to vSAN/ESXi 7.0 U3f 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 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

• Significant performance degradation of vSAN cluster
• VM operations impacted