For vSAN stretched cluster functionality on VxRail, vSphere Distributed Resource Scheduler (DRS) is highly recommended. DRS provides initial placement assistance, and automatically migrates virtual machines to the corrected site in accordance with the Host and VM affinity rules. It can also help relocate virtual machines to their correct site when a site recovers after a failure.

DRS awareness of vSAN stretched clusters is built into vSAN 7 Update 2. There is no need to make changes in configuration or operating processes. It works with all affinity rules. The recommendation with previous versions of vSAN was to set DRS Automation to Manual or Partially Automated. With vSAN 7 Update 2, this recommendation can be set to Fully Automated to get the most benefits from DRS with vSAN stretched clusters.

vSphere’s DRS, tightly integrated with vSAN Update 2, introduces a fully automated read locality solution for recovering from failures on a vSAN stretched cluster. The read locality information indicates the hosts to which the VM has full access, and DRS uses this information when placing a VM on a host on vSAN Stretched Clusters. DRS migrates the VMs back to the primary site when the vSAN resynchronization is completed and the VMs’ data components have achieved full read locality. This DRS can be placed in fully automatic mode to react to site failures. 

In case of partial site failures, if a VM loses read locality due to loss of data components equal to or greater than its failure to tolerate, vSphere DRS will identify the VMs that consume an exceedingly high read bandwidth and try to rebalance them to the secondary site. This way, we ensure that VMs with read-heavy workloads are not impacted during partial site failures. When the primary site is back online and the data components have completed resynchronization, the VM is moved back to the site to which it is connected.

Note: We support vSAN stretched clusters with vSphere Standard Edition. vSphere Enterprise plus is required for DRS.