vSAN clusters must be made up of physical hosts that contain either a combination of mechanical disks and flash devices (hybrid configuration) or all flash devices (all-flash configuration) that contribute cache and capacity to the vSAN distributed datastore.

In a hybrid configuration, one flash device and one or more mechanical drives are configured as a disk group, with a disk group being able to maintain a maximum of seven mechanical drives. One or more disk groups are utilized in a vSphere host. In a hybrid configuration, the flash device serves as read-and-write cache for the vSAN datastore, while the mechanical drives make up the capacity aspect of the datastore. By default, vSAN uses 70 percent of the flash capacity as read cache and 30 percent as write cache. It is typically not recommended to modify this ratio.

In an all-flash configuration, the flash device in the cache tier is used for write caching only (no read cache) because read performance from the capacity flash devices is more than sufficient for even the most demanding enterprise application. In an all-flash configuration, two different grades of flash devices are commonly used, a lower capacity, higher endurance device for the cache layer, and more cost effective, higher capacity, lower endurance devices for the capacity layer. Writes are performed at the cache layer and then de-staged to the capacity layer, only as needed. This helps extend the usable life of the lower endurance flash devices in the capacity layer.