Understanding vSAN Disk Groups


VMware vSAN is enterprise-class storage for hyper-converged infrastructure (HCI). Native to the vSphere hypervisor, vSAN delivers flash-optimized, secure storage. It utilizes commodity x86 server components to lower costs up to 50% versus traditional server and storage array architectures.

A wide variety of deployment and configuration options make vSAN a flexible and highly scalable HCI storage solution. A single vSAN cluster consists of any number of physical server hosts from 2 to 64.

One of the core component of VMware vSAN is Disk Group.

vSAN Disk Groups are like a logical unit or collection of local disk drives of hosts who participating in vSAN datastore. Each host contains flash devices (all flash configuration) or a combination of magnetic disks and flash devices (hybrid configuration) that contribute cache and capacity to the vSAN distributed datastoreSo Disk Groups is like a container, where the SSD for cache and capacity devices (SSD or HDDs) are in relation. On each ESXi host, 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). A disk group can have up to 7 seven magnetic disks and an ESXi host can have up to 5 disk groups. 

So if we do a quick math, you can have a maximum of 40 disks in a single esxi host contributing storage to vSAN datastore. Quite good amount of storage and performance (think they can be all flash devices!)  🙂


VMware vSAN uses an aggregation of this disk groups to create a single datastore when you enable vSAN.

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.

There are 2 types of vSAN Cluster architecture, Hybrid vSAN and All-Flash vSAN.

– Hybrid vSAN cluster can have up to 64 ESXi hosts and each hosts will have one or more disk group up to the maximum of 5 disk groups. Each disk group in the Hybrid vSAN should contain one Flash device (cache Tier – SSD) and one or more Magnetic disk (Max 7) (Capacity tier – HDD) to form a storage capacity. It provides optimal performance with 70% of flash used for read caching and 30% for Write cache from the cache tier. Magnetic disks will be purely used for capacity. and the flash devices serve as a read cache and a write buffer.


– All-Flash vSAN cluster can have up to 64 ESXi hosts and each host will have one or more disk group up to the maximum of 5 disk groups. Each disk group in the All-Flash vSAN hosts should contain one Flash device (cache Tier – SSD) and one or more Flash disk (Max 7) (Capacity tier – SSD) to form a storage capacity. All-Flash vSAN uses SSD Flash disks for both cache tier and Capacity tier. It provides High Performance and suitable for heavy I/O Workloads. Since both Cache and Capacity tier is Flash device, Entire Cache tier 100% is used for write cache not utilized for read cache. In all-flash clusters, all read requests come directly from the flash pool capacity.

Just a thing to know only local or direct-attached (DAS) capacity devices can participate in a vSAN cluster. vSAN cannot consume other external storage, such as SAN or NAS, attached to cluster.

vSAN clusters can include hosts with or without capacity devices. The minimum requirement is three hosts with capacity devices. For best results, create a vSAN cluster with uniformly configured hosts.


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