Review: VMware Virtual SAN turns storage inside-out

VSAN architecture
It's important to understand how VSAN works in comparison to other "software-defined storage" solutions. First and foremost is the fact that VSAN is tightly integrated with the ESXi kernel. Tight integration with ESXi means that storage is provided directly through the hypervisor, and not via a separate VM (as in the case of Nutanix, for example). It also means that VSAN requires no additional software installation beyond VMware vSphere (that is, the ESXi hypervisor) and VMware vCenter Server.

Another important architectural point concerns the hardware requirements for the cluster. VSAN requires a minimum of three nodes in order to form the cluster and will support up to 32 nodes. With three nodes, you can tolerate a single node failure and still have a cluster of two. While the system will run in a degraded mode, you can't create the cluster without three nodes present. The default autoprovision mode creates a single VSAN data store that consists of all available space on all unprovisioned hard disks on each node.

Each node in the cluster must have a minimum of one hard disk drive and one solid-state drive in order to join the cluster. These disks must not be provisioned prior to joining the VSAN cluster and must be individually addressable. This means the disk controller or HBA (host bus adapter) must be configured in pass-through mode or each drive must be individually configured in RAID 0 mode. One last point to be aware of here is that the VSAN data store will only be available to nodes that are part of the VSAN cluster.

VSAN supports SAS, near-line SAS, and SATA hard disk drives, as well as SAS, SATA, and PCI Express solid-state drives. You'll want to check the VSAN hardware compatibility list to determine if a specific device has been tested. The inclusion of SATA opens up a wide range of devices for use in a VSAN cluster.

On each host or node, the drives are arranged into as many as five disk groups with one SSD and up to seven HDDs in each group. The SSD in each disk group acts as a caching tier; it does not contribute to the total capacity of the data store. VSAN stores everything on the clustered file system as an object, so is similar in that respect to the Nutanix solution (see my Nutanix review).

User settings in VSAN 1.0 are kept to a minimum:

Number of failures to tolerate. This is the number of concurrent host, network, or disk failures the cluster will tolerate and still ensure the availability of the object (such as a virtual machine disk). This value defaults to one, meaning the system will only tolerate a single failure. Increasing that number requires more hardware.

Number of disk stripes per object. This value defines the number of physical disks across which each replica of a storage object is striped. It defaults to one. Setting this value to greater than one might increase performance (when a request results in a cache miss), but it's not guaranteed.

Flash read cache reservation. This is the amount of flash capacity reserved on the SSD as read cache for the storage object with a default of zero. The VSAN scheduler handles cache allocation by default, although it is possible to increase the amount on an object basis to address performance issues.