On each ESXi host that contributes its local disks to a Virtual SAN cluster, disks are organized into disk groups. A disk group is a main unit of storage on a host. Each disk group includes one SSD and one or multiple HDDs (magnetic disks).
Virtual SAN uses an aggregation of disk groups to back up a single datastore that is created when you enable Virtual SAN.
In the disk group, the SSD primarily serves as a read cache write buffer, while the HDDs are used for permanent storage. Typically, a higher SSD to HDD ratio, both in size and quantity, improves performance.
Depending on the mode you select when enabling Virtual SAN on a cluster, you can use different ways to organize disks into groups.
Virtual SAN claims all available and usable disks and organizes them into default groups with one SSD and one or multiple HDDs. If you add more disks to hosts or add new hosts to the Virtual SAN cluster, all applicable disks are claimed by Virtual SAN. Virtual SAN in automatic mode claims only local disks on the ESXi hosts in the cluster. You can add any remote nonshared disks manually.
You must specify hosts and disks on the hosts to be used for the Virtual SAN datastore. You have two methods of organizing disks into disk groups, semi-automatic and manual.
When you use the semi-automatic method, Virtual SAN organizes the disks that you specify into default disk groups.
Another option is to manually create user-defined disk groups and select disks for each group. When you create a disk group manually, your main consideration should be the ratio of SSD to Raw HDD capacity. Although the ratios depend on use cases and workloads, the best practice is to use SSD capacity of at least 10 percent of the total consumed HDD capacity in each disk group, without counting the protection copies. For example, if the size of your Raw HDD capacity on the disk group is 4TB, the recommended SSD capacity is 400GB.
Virtual SAN disks cannot be used by other features, such as RDM, VMFS, diagnostic partition, and so on.