Dell VxRail Architecture Overview

VMware vSAN OSA overview

The initial release of VMware vSAN was based on VMware OSA in which a hard disk drive was used. The solid state drives were expensive with low capacity, CPU core counts were in single digits, and 10 GB Ethernet was just being implemented.

The building block of VMware vSAN OSA is the disk group which consists of a single cache drive that is partnered with a collection of capacity drives. The cache drives are based on high-performing SSDs or NVMe technology, while the capacity drives can be either solid state drive or hard drive. The characteristics of the capacity drive in a disk group differ from the cache drives as they typically have a larger storage capacity. A VMware vSAN database which is based on the VMware vSAN OSA, consists of a collection of disk groups.

VMs never directly interact with any storage devices at the capacity layer. The capacity tier has slightly different roles for hybrid and all-flash configurations. In hybrid configurations, the cache device acts as a read cache and write buffer. This dramatically improves the performance of the I/O while allowing for scale-out based on low-cost SATA or SAS disk drives. Since the capacity disks are also flash in all-flash, there is no need to move blocks between flash devices. For this reason, VMware vSAN dedicates 100 percent of the device to write buffering.

Overall performance of VMware vSAN OSA is based on the number of disk groups in the data store, and also whether the capacity drives are solid state drives or the low performing hard drives. Availability is based on the specific enforcement setting in a VMware vSAN storage policy. This setting dictates whether a copy of an I/O write operation is distributed across storage devices and also across disk groups to support availability requirements.