There are some specifics about remediating hosts that are part of a Virtual SAN cluster.
By design only one host from a Virtual SAN cluster can be in a maintenance mode at any time. Because of that the host remediation process might take extensive amount of time to complete since Update Manager must handle the remediation of the hosts sequentially. Update Manager remediates hosts that are part of a Virtual SAN cluster sequentially even if you select the option to remediate them in parallel.
You have several ways to handle remediation of a host that is part of a Virtual SAN cluster, depending on how you want the virtual machines handled on the host:
You can put the host in maintenance mode from the vSphere Web Client, and remediate the host by using Update Manager.
You can put the host in maintenance mode from the vSphere Client, and remediate the host by using Update Manager.
You can have a host entering maintenance mode during the Update Manager remediation process.
From the vSphere Web Client you can select between multiple options when putting a host from a Virtual SAN cluster in maintenance mode: Ensure accessibility, Full data evacuation, and No data evacuation. The Ensure accessibility option is the default option, and means that when you put a host in maintenance mode, the Virtual SAN ensures that all accessible virtual machines on this host remain accessible. To learn more about each of the options, see the Place a Member of Virtual SAN Cluster in Maintenance Mode topic from vSphere Storage guide.
When you put a host from a Virtual SAN cluster into maintenance mode from the vSphere Client, you must confirm a maintenance mode warning message. Before confirming the message, you can select to move powered off and suspended virtual machines to other hosts in the cluster, but you have no options on how to handle the powered on virtual machines on the host. The powered on virtual machines are automatically handled equivalently to the default Ensure accessibility option.
When you use the Update Manager, the remediation process might put the host from the Virtual SAN cluster in maintenance mode, which would handle the virtual machines on the host in the manner of the default Ensure accessibility option.
If a host is a member of a Virtual SAN cluster, and any virtual machine on the host uses a VM storage policy with a setting for "Number of failures to tolerate=0", the host might experience unusual delays when entering maintenance mode. The delay occurs because Virtual SAN has to migrate the virtual machine data from one disk to another in the Virtual SAN datastore cluster. Delays might take up to hours. You can workaround this by setting the "Number of failures to tolerate=1" for the VM storage policy, which results in creating two copies of the virtual machine files in the Virtual SAN datastore.