If you are familiar with thin vs thick provisioning disk, this post will help you understand much easier about the OSR (Object Space Reservation).
By default, virtual machine storage objects deployed on vSAN are thin provisioned. OSR is one of the storage policy, that specifies the percentage of the logical size of storage object that should be reserved (thick provisioned) when the virtual machine is being provisioned. The rest of the storage object will remain thin provisioned. The default value is 0 percent, implying the object is deployed as thin. The maximum value is 100 percent, meaning that the space for the object is fully reserved, which can be thought of as full i.e., thick provisioned. Since the default is 0 percent, all virtual machines deployed on vSAN are provisioned as thin disks unless you state a requirement for ObjectSpaceReservation in the policy. If OSR is specified with a specific percentage, a portion of the storage object associated with that policy is reserved.
For ex : Let’s consider the below pic where the OSR is set to 50% and so as the half amount of storage is thick and remaining half is thin.
There is no eager-zeroed thick format on vSAN. OSR, when used, behaves similarly to lazy-zeroed thick. There are several safeguards that will prevent overcommitment. For instance, if there is not enough storage capacity across the required number of hosts in the cluster to satisfy a replica or a stripe width policy setting, then vSAN will show warnings in vSphere Web Client.
Object space reservation changes the how vSAN handles thickness for the VMDK. If object space reservation is increased on the fly, for example, while moving from 5 percent to 10 percent object space reservation (which would mean creating a new policy) and the policy is then applied to the virtual machine, a new object will then be created and a resync will be required.
Capacity overview in vSan monitor:
Used VM Overreserved displays how much space has been reserved using Object Space Reservation that is not being used by the virtual machine.