For ESXi hosts in a cluster, the remediation process is sequential by default.
When you remediate a cluster of hosts sequentially and one of the hosts fails to enter maintenance mode, vSphere Lifecycle Manager reports an error and the remediation process stops and fails. The hosts in the cluster that are remediated stay at the updated level. The ones that are not remediated after one host fails remain unupdated.
The host upgrade remediation of ESXi hosts in a cluster proceeds only if all hosts in the cluster can be upgraded.
If you initiate remediation at a data center level, the remediation processes for the clusters run in parallel. Clusters that you manage with a single vSphere Lifecycle Manager image are not remediated against the attached baselines or baseline groups. If the remediation process fails for one of the clusters within a data center, the remaining clusters are still remediated.
Before you start remediation, you can generate a report that shows which cluster, host, or virtual machine has the cluster features enabled. For more information, see Remediation Pre-Check Report.
Remediation and Cluster Settings
If a host in a DRS-enabled cluster runs a virtual machine on which vCenter Server is installed, DRS first attempts to migrate the virtual machine running vCenter Server to another host, so that the remediation succeeds. If the virtual machine cannot be migrated to another host, the remediation fails for the host, but the remediation process for the cluster does not stop. vSphere Lifecycle Manager proceeds to remediate the next host in the cluster.
Remediation of hosts in a cluster requires that you temporarily disable cluster features such as VMware DPM and HA admission control. Also, you must turn off Fault Tolerance if it is enabled on any of the virtual machines on a host, and disconnect the removable devices connected to the virtual machines on a host, so that they can be migrated with vMotion. For more information about configuring the vSphere Lifecycle Manager remediation settings, see Configuring the vSphere Lifecycle Manager Remediation Settings.
If a vCenter HA failover is initiated during the remediation of a cluster, the remediation task is canceled. After the failover finishes, you must restart the remediation task on the new node.
When you perform remediation on a cluster that consists of not more than two hosts, disabling HA admission control might not be enough to ensure successful remediation. You might need to disable vSphere High Availability (HA) for the cluster. If you keep HA enabled, the remediation attempts on hosts in the cluster fail, because HA cannot provide recommendation to vSphere Lifecycle Manager to place any of the hosts into maintenance mode. The reason is that if one of the two hosts is placed into maintenance mode there is no failover host left available in the cluster. To ensure successful remediation on a two-node cluster, you must disable HA for the cluster or place the hosts in maintenance mode manually and then remediate the two hosts in the cluster.
vSphere Lifecycle Manager remediates hosts that are part of a vSAN cluster sequentially. The reason is that by design only one host from a vSAN cluster can be in a maintenance mode at any time. For more information about using vSphere Lifecycle Manager with vSAN clusters, see vSAN Clusters and vSphere Lifecycle Manager.