Part 1 – Introduction to VSAN

VSAN is the most talked about feature these days and there are around 5000 VSAN customers today as Duncan Epping pointed out on his blog. So i wanted to get my hands dirty with the new version 6.2 supporting many advanced features. So let’s get started.

Click on the links below to follow my other posts related to VSAN

  1. Introduction to VSAN
  2. VSAN Lab Setup
  3. Upgrade On-Disk Format Version
  4. Disk Groups and vsanDatastore
  5. Storage Providers and Storage Profiles
  6. Space Efficiency Techniques – VSAN
  7. Using esxcli commands – VSAN

VSAN is the storage virtualization solution from VMware tightly coupled with ESXi to enable local storage to be used as storage pools which can then be assigned to virtual machines. Local storage? Yes, local storage can be used as datastores and yes all the traditional vSphere features that we love like HA, vMotion and DRS still work. VSAN is a cluster level feature like HA and DRS, connected by a specialized network called VSAN network.

For the VSAN to work, each host participating in the VSAN cluster has to contribute at least a flash device for cache and atleast one magnetic device for the data. VSAN can be implemented as hybrid or all-flash cluster. In Hybrid cluster, flash devices are used for cache and magnetic disks are used for data.Hosts without any local storage can also participate it the VSAN cluster provided a minimum of 3 ESXi hosts contributing storage capacity are present in the cluster. Flash devices designated for caching cannot be used for capacity and do not contribute to the datastore capacity. Below is the VSAN architecture.

VSAN Arch.jpg

© VMware

Disk Group

A disk group is a collection of physical devices contributed to the VSAN cluster by each individual ESXi hosts. Each disk group must have a dedicated caching device and one or more capacity devices. Caching device of a disk group cannot be shared with other disk groups. An ESXi host can use multiple disk groups.

  • Max number of disk groups per host is 5
  • Max number of magnetic disk per disk group is 7 and each disk group can have only one SSD.


An object is a logical volume which has its data distributed across the cluster. Objects are created when a VM is provisioned on a VSAN datastore based on the storage policy defined on the VM, it also creates a VM home namespace which stores all the metadata files of the VM

Storage Policy Based Management

A VM deployed on the VSAN must have a policy assigned to it. A policy defines the requirements of a VM based on the perfomance and availability. If no policy is defined then the default storage policy is applied which has Number of failures to tolerate as 1, single disk stripe for each object and a thin provisioned disk.

On-Disk Format

VMware acquired Virsto long back and VSAN uses their log-based File System capable of perfoming snapshots and clones effectively. VSAN 6.2 now supports on-disk virtual format 3.0. More information about Virsto here and here. Thanks Duncan Epping for explaining things really well.

VSAN Features

  • Shared Storage Support
  • On-Disk Format
  • Fault Domains
  • Stretched Clusters
  • VSAN Health Service
  • Integration with vSphere storage features
  • VM Storage policies

VSAN Requirements

So what does it take to have VSAN implemented in our infrastructure? Below are those.

  • An SSD or PCIe Flash disk on each host for cache, cache disks must not be formatted with VMFS
  • A Flash or magnetic disk for VM data storage.
  • A minimum of 3 hosts that contribute cache and capacity devices is required to be part of a VSAN cluster.
  • Each host in the cluster must have a VMkernel interface to handle the VSAN traffic.
  • Multicast  must be enabled on physical switches and routers
  • Acquire license for VSAN

Limitations of VSAN

  • A host can participate in only one VSAN cluster at any point of time.
  • VSAN does not support DPM, SCSI reservations, RDM and SIOC.

Hope this post was informative. Thanks!

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