There are some specifics about remediating hosts that are part of a vSAN cluster.
Maintenance Mode Specifics of vSAN Clusters
By design only one host from a vSAN 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 vSAN cluster sequentially even if you select the option to remediate them in parallel.
If the vSAN cluster has a system-managed baseline attached by default, you can remediate the cluster against the baseline to bring all the hosts in a compliant state, and install the latest software recommended by vSAN.
You have several ways to remediate a host that is part of a vSAN cluster, depending on how you want the virtual machines handled on the host:
You can put the host in maintenance mode manually, 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 vSAN 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 vSAN 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 vSAN Cluster in Maintenance Mode topic from vSphere Storage guide.
When you put a host from a vSAN cluster into maintenance mode, 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 Update Manager, the remediation process might put the host from the vSAN cluster in maintenance mode, which handles the virtual machines on the host in the manner of the default Ensure accessibility option.
If a host is a member of a vSAN 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 vSAN has to migrate the virtual machine data from one disk to another in the vSAN datastore cluster. Delays might take up to hours. You can work around 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 vSAN datastore.
vSAN Health Check
Update Manager performs a pre-remediation check of vSAN clusters to ensure a successful remediation. If the vSAN health check is successful, you can continue with the remediation process.
If some of the tests against the vSAN cluster fail, Update Manager displays the vSAN health check as unsuccessful in the remediation pre-check dialog box, and recommends actions that you take before you remediate the cluster.
Running the pre-remediation check part of which is the vSAN Health check does not prevent you from starting the remediation process on a vSAN cluster. However, it is best to wait for the results from the vSAN Health check in case you need to take extra actions to ensure successful remediation on the cluster.
If you do not take the recommended actions from the vSAN Health check, you can still proceed to of the vSAN cluster or a host from the cluster. Update Manager successfully puts the host in maintenance mode, patches, or upgrades the host successfully. However, the process might fail to exit the host from maintenance mode, and the remediation process eventually fails. The host from the vSAN cluster is upgraded, but you must take manual steps to put the host out of maintenance mode.
As part of the upgrade process, the vSAN health check runs before the host enters maintenance mode and after the host exits maintenance mode.
For more detailed information about vSAN Health, select a vSAN cluster, click the Monitor tab, and click .
In the vSphere Client, the remediation pre-check is available from the Updates tab when you select a host or a cluster from the inventory.
In the vSphere Web Client, the remediation pre-check is available from the last page of the remediation wizard.