vSAN/DRS awareness to be introduced in vSAN/vSphere 7.0!

It was briefly mentioned here , but I figured I would elaborate on this new cool feature for vSAN Stretched Clusters which is DRS Awareness of vSAN Stretched Clusters. So what does this mean? Well, it is fairly straight forward. DRS will take vSAN resync traffic into consideration when the DRS algorithm runs. I can probably explain best by talking through a scenario:

• vSAN Stretched Cluster environment with 4 hosts and a witness
• VMs running in Preferred and in Secondary
• VMs configured with “should rules” to stay within their fault domain
• ISL between “data locations” is impacted
• HA has restarted the VMs of the secondary site in the preferred site
• ISL is now restored

What would happen without DRS awareness of vSAN stretched clusters is that DRS would automatically migrate VMs back to the Secondary site as soon as it becomes available. DRS runs every minute in vSphere 7.0 so it is very likely that vSAN is still resyncing data. The problem with this is two-fold:

1. The vMotion process will slow down the resync of data temporarily
2. Blocks which have not been resynced and are being read by the VM will need to be fetched from the remote location

As you can imagine this is an undesired situation. As such in vSphere / vSAN 7.0 a whole new level of integration is introduced between DRS and vSAN. Now DRS will be aware of what is happening on the vSAN layer. If vSAN is syncing a particular component of a virtual machine, then DRS will not move the VM back! It will wait until the resync has completed and then move the VM back. This ensures that the migration won’t conflict with the resync, and of course that when the VM is migrated that it will have “site read locality”.

It is a feature our team had been asking for and which was tested within VMware Cloud on AWS, and I am happy to see it made it into the “regular” vSphere release.