VMware vSAN in a nutshell

What is vSAN ?

Virtual SAN is a new feature introduced in vSphere 5.5. It is a hypervisor-converged storage solution built by aggregating the local storage attached to the ESXi hosts managed by a vCenter.

What should I know before building a vSAN ?

  • A minimum of 3 hosts are required.
  • The host must have a SSD and Magnetic HDD.
  • VMware recommends a 10Gb network for configuring vSAN but you can also use 1Gb network for the same.

Coming to the concept of vSAN

The VMs stored in vSAN will use the SSD for performance and HDD for storage. You can add  a maximum of 1 SSD and 7 HDDs for creating a single vSAN diskgroup. Anything more than that would require another diskgroup. You can create a maximum of 5 diskgroups for a host. A seperate vmkernel network has to be created for the Virtual SAN traffic. This interface will be used for the inter host communications as well as for read and write operations whenever a vSphere host in the cluster is the owner of a particular virtual machine but the actual data blocks making up that virtual machine’s objects are located on a remote host in the cluster.

What about the redundancy ?

Here comes the beauty of this technology.Yes vSAN has redundancy. Whenever a VM is created in vSAN, in addition to its vmdk, a replica of that vmdk along with a witness disk (rewind the concepts of MS clustering) will be created. The witness disk will handle the switch over of vmdk at the time of a host failure. To ensure high availability of the VM, these two vmdks and witness disk will be stored in different hosts. Whenever there is a write IO, the operation will be performed on both the vmdk present in different hosts. The SSD in each host which acts as the write buffer will take care of this activity.

If the storage goes down: 

The vmdk in that host will be marked as 'degraded' and a mirror of that vmdk will be created in another host. If there is no sufficient resources for the mirroring, vSAN will wait until a storage is added.

If the host goes down:

Suppose a host which holds a vmdk of the VM goes down, the vmdk in that host will be marked as 'absent' and vSAN waits for 60 minutes before it creates a mirror of that vmdk in another host. If the host comes back before 60 minutes, the changes will be synchronized.  

In both scenarios VM will be unaware of these activities.

Working with other VMware technologies ?

Virtual SAN can be configured only via vSphere Web Client. It works well with HA,DRS & vMotion but does not support FT, DPM, Storage DRS or Storage I/O Control.


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