In this post, we talk about a particular behaviour with using the default (or None) policy with VSAN. I have stated many times in the past that when a VM is deployed on the VSAN datastore, it behaves like it is thinly provisioned unless the capability ‘Object Space Reservation’ (OSR) is specified in the VM Storage Policy. The OSR will pre-allocate space on the VSAN datastore for the virtual machine’s storage objects, and is specified as a percentage of the actual VMDK size. However, there is a slightly different behaviour when the default policy is used. Once again, I was in a conversation with a customer who stated that when he used the default policy of “None”, he could see space being consumed on the VSAN datastore was equal to the size of the VMDK * FTT (Number of Failures To Tolerate). He wondered why this was the case when the default policy clearly did not contain an Object Space Reservation capability.
In a nutshell, if no policy is chosen, and the default policy (None) is used, VMs deployed on VSAN can be specified as thick or thin. This can be done by expanding the Hard Disk configuration options in the VM deploy wizard customize hardware as shown below. Here, in the Disk Provisioning section, I have selected Thin Provision, but by default it will have LZT (Lazy Zero Thick) selected: