One of the Best Practices for VSAN is to not use the VSAN Datastore for the Scratch Partition or for the Syslog Server.
As per VMware KB:
Creating a persistent scratch location for ESXi 4.x and 5.x (1033696)
Note: It is not supported to configure a scratch location on a VSAN datastore.
So what is the reason for this ?
Imagine that for any reason you are forced to leave an ESXi from the VSAN Datastore, and when you type the command:
~ # esxcli vsan cluster leave
You will get this error:
/dev/disks # esxcli vsan cluster leave
Failed to leave the host from VSAN cluster. The command should be retried (Sysinfo error on operation returned status : Failure. Please see the VMkernel log for detailed error information
[vob.sysinfo.set.failed]: Sysinfo set operation VSI_MODULE_NODE_umount failed with error status Failure.
Basically, the ESXi host can’t leave the Cluster as it’s impossible to release the configured scratch partition that is locked in the VSAN Datastore.
This can also happen if the Syslog folder is configured inside the VSAN Datastore.
To solve this just connect directly to your host using the vSphere Client and change the Scratch Partition to another folder.
Follow the Best practices and don’t ever configure the Scratch Partition or the Syslog on the VSAN Datastore.