Below is a screenshot taken from the Cluster > Configure > vSAN > Services > Advanced Options view, which highlights the new Large Cluster Support setting. It also highlights a second new setting which is Automatic Rebalance. We will cover that in a moment.
Note however, that if you decide to enable Large Cluster Support, each of the hosts in the cluster will need to be rebooted. Both the message associated with the Advanced Option, and the message displayed once the option is enabled, highlight this requirement.
Until the hosts have been rebooted, the vSAN extended configuration in sync health check will report an error, stating that a host reboot is recommended.
A nice feature to simplify support for large vSAN clusters that are > 32 nodes. [Note: I’ve since been told that this option is also available in vSAN 6.7 U2, although I have to admit that I don’t remember seeing it in that version of vSAN].
Let’s now talk about the second Advanced Option that we saw earlier – Automatic Rebalance. This is related to vSAN object components, which are distributed around the vSAN cluster in various disk groups and disks. Automatic Rebalance is there to ensure that no part of the system is overloaded, while other parts are very lightly loaded. If Advanced Option is enabled, and the algorithms associated with Automatic Rebalances determines that the system is unbalanced, it will kick off rebalancing automatically. The threshold to initiate rebalance is set to 30% by default, which means that if any two disks have this variance (one is 30% more loaded than the other), rebalancing of components begins. Rebalancing will continue until the variance reaches half of the set threshold value, i.e. 15% by default (or until Automatic Rebalance is disabled).
There is also a health check for vSAN Disk Balance, where you can see disk usage details of the vSAN cluster. If Automatic Rebalance is enabled, vSAN automatically tries to keep this health check green. If it is disabled, this health check is triggered and will requires the admin to manually trigger a Rebalance Disks task, or re-enable Automatic Rebalance.
Another very useful advanced option, and one customers should consider enabling on their vSAN 6.7U3 deployments.