When you use Virtual SAN, View defines virtual machine storage requirements, such as capacity, performance, and availability, in the form of default storage policy profiles, which you can modify. Storage is provisioned and automatically configured according to the assigned policies.
The default policies that are created during desktop pool creation depend on the type of pool you create. The names of the policies are OS_DISK (for operating system files), PERSISTENT_DISK (for user data files), REPLICA_DISK (for replicas), and VM_HOME (for virtual machine files such as .vmx and .vmsn files). For example, a REPLICA_DISK policy is created only for linked-clone pools. Changes to the policy are propagated to newly created virtual machines and to all existing virtual machines in the desktop pool.
Virtual SAN offers a storage policy framework so that you can control the behavior of various virtual machine objects that reside on the Virtual SAN datastore. An example of an object in Virtual SAN is a virtual disk (VMDK) file, and there are four characteristics of each object that are controlled through policy:
Stripes: Number of stripes of data. The number of disk stripes affects how many magnetic disks you have (HDDs).
Resiliency: Number of failures to tolerate. The number of host failures to tolerate depends, of course, on the number of hosts you have.
Storage Provisioning: Thick or Thin.
Cache Reservation: Read-cache reservation.
The stripes and cache reservation settings are used to control performance. The resiliency setting controls availability. The storage provisioning setting control capacity. These settings, taken together, affect how many vSphere hosts and magnetic disks are required.
For example, if you set the number of disk stripes per object to 2, Virtual SAN will stripe the object across at least 2 HDDs. In conjunction with this setting, if you set the number of host failures to tolerate to 1, Virtual SAN will create an additional copy for resiliency and therefore require 4 HDDs. Additionally, setting the number of host failures to tolerate to 1 requires a minimum of 3 ESXi hosts, 2 for resiliency and the third to break the tie in case of partitioning.
If you inadvertently attempt to use settings that contradict each other, when you attempt to apply the settings, the operation will fail, and an error message will tell you, for example, that you do not have enough hosts.
There is no requirement for any user action associated with these default policies. Policies are created for both linked-clone desktop pools and full-clone desktop pools.
You can use either the vSphere Command-Line Interface (esxcli) or the vSphere Web Client to change the default storage policy profiles. Each virtual machine maintains its policy regardless of its physical location in the cluster. If the policy becomes noncompliant because of a host, disk, or network failure, or workload changes, Virtual SAN reconfigures the data of the affected virtual machines and load-balances to meet the policies of each virtual machine.