Here is another tidbit on how you can leverage the power of vSphere Events, which now includes over 2K+ as of vSphere 8.0 Update 1 to help monitor when an ESXi filesystem and/or partition is low on disk space.
With vSphere 6.7 or later, we have two events that you can use to help alert when either an ESXi ramdisk (e.g. /var) or VFAT partition (e.g. bootbanks) has filled up.
- Ramdisk: esx.problem.visorfs.ramdisk.full
- VFAT: esx.problem.vfat.filesystem.full.other
When either of these occur, you can easily find them under the Monitor->Events section for an ESXi host as shown in the screenshot below.