When you set up or import a vSphere Lifecycle Manager image to use with a cluster, the software specified in the image is not immediately installed on the hosts in that cluster. To apply the software specification from the image to the hosts, you must remediate the cluster against that image.
To initiate remediation of a cluster, you must have the required privileges. For a list of all vSphere Lifecycle Manager privileges and their descriptions, see vSphere Lifecycle Manager Privileges For Using Images. For more information about managing users, groups, roles, and permissions, see the vSphere Security documentation
During the remediation of a cluster against a vSphere Lifecycle Manager image, the ESXi hosts in the cluster are remediated sequentially. So, if the remediation for a single host in the cluster fails, the remediation of the entire cluster stops. Parallel remediation is an option only supported if you manage a cluster with vSphere Lifecycle Manager baselines.
During remediation, the image that you set up for the cluster is installed on all ESXi hosts in the cluster.
When you remediate a cluster that contains a single ESXi host or that has vSphere Storage DRS disabled or in manual mode, the remediation process cannot put that host into maintenance mode. So, to proceed with the remediation, you must power off the virtual machines that are running on the host, move them to another host, or select a user policy that allows the remediation process to power off the virtual machines. You can also set a user policy to power on the virtual machines after the host is remediated.
For vSAN clusters, the remediation operation includes a hardware compatibility check. Depending on how you configure the vSphere Lifecycle Manager remediation settings, vSphere Lifecycle Manager might not proceed with the remediation task if hardware compatibility issues exist. For information about configuring the global vSphere Lifecycle Manager remediation settings, see Configure Remediation Settings for vSphere Lifecycle Manager Images. For information about configuring the remediation settings for a particular cluster, see Edit the Remediation Settings for a Cluster.
If the update requires it, hosts are put into maintenance mode before remediation. Virtual machines cannot run when a host is in maintenance mode. To ensure a consistent user experience,vCenter Server migrates the virtual machines to other hosts within the cluster before a host is put into maintenance mode. vCenter Server can migrate the virtual machines if the cluster is configured for vMotion and if DRS and VMware Enhanced vMotion Compatibility (EVC) are enabled. EVC guarantees that the CPUs of the hosts are compatible, but it is not a prerequisite for vMotion.
You can configure vSphere Lifecycle Manager to disable HA admission control for the cluster before remediation. However, disabling HA admission control before you remediate a two-node cluster that uses a single vSphere Lifecycle Manager image causes the cluster to practically lose all its high availability guarantees. The reason is that when one of the two hosts enters maintenance mode, vCenter Server cannot failover virtual machines to that host and HA failovers are never successful. For more information about HA admission control, see the vSphere Availability documentation.