The disk is not full – but getting disk space utilization alerts

snapd uses loop devices and they are listed as 100% use. To get zimbra to ignore loop devices

su - zimbra 
zmlocalconfig -e zmstat_df_excludes='/dev/loop0:/dev/loop1:/dev/loop2:/dev/loop3:/dev/loop4:/dev/loop5:/dev/loop6'
zmstatctl restart 

References
https://wiki.zimbra.com/wiki/Disable_Disk_Space_Monitoring_for_Loop_Devices
https://forums.zimbra.org/viewtopic.php?t=5729