vsphere-esxi-vcenter-server-55-storage-guide

Chapter 19 Working with Virtual SANTable 19‐1. vSphere HA networking differencesVirtual SAN EnabledVirtual SAN DisabledNetwork used by vSphere HA Virtual SAN storage network Management networkHeartbeat datastoresHost declared isolatedAny datastore mounted to > 1 host,but not Virtual SAN datastoresIsolation addresses not pingable andVirtual SAN storage networkinaccessibleAny datastore mounted to > 1 hostIsolation addresses not pingable andmanagement network inaccessibleIf you change the Virtual SAN network configuration, the vSphere HA agents do not automatically pick upthe new network settings. So to make changes to the Virtual SAN network, you must take the followingsteps in the vSphere Web Client:1 Disable Host Monitoring for the vSphere HA cluster.2 Make the Virtual SAN network changes.3 Right-click all hosts in the cluster and select Reconfigure HA.4 Re-enable Host Monitoring for the vSphere HA cluster.Capacity Reservation SettingsWhen you reserve capacity for your vSphere HA cluster with an admission control policy, this setting mustbe coordinated with the corresponding Virtual SAN setting that ensures data accessibility on failures.Specifically, the Number of Failures Tolerated setting in the Virtual SAN rule set must not be lower than thecapacity reserved by the vSphere HA admission control setting.For example, if the Virtual SAN rule set allows for only two failures, the vSphere HA admission controlpolicy must reserve capacity that is equivalent to only one or two host failures. If you are using thePercentage of Cluster Resources Reserved policy for a cluster that has eight hosts, you must not reservemore than 25% of the cluster resources. In the same cluster, with the Host Failures Cluster Tolerates policy,the setting must not be higher than two hosts. If less capacity is reserved by vSphere HA, failover activitymight be unpredictable, while reserving too much capacity overly constrains the powering on of virtualmachines and inter-cluster vMotion migrations.Virtual SAN and vSphere HA Behavior in a Multiple Host Failure SituationAfter a Virtual SAN cluster fails with a loss of failover quorum for a virtual machine object, vSphere HAmight not be able to restart the virtual machine even when the cluster quorum has been restored. vSphereHA guarantees the restart only when it has a cluster quorum and can access the most recent copy of thevirtual machine object. The most recent copy is the last copy to be written.Consider an example where a Virtual SAN virtual machine is provisioned to tolerate one host failure. Thevirtual machine runs on a Virtual SAN cluster that includes three hosts, H1, H2, and H3. All three hosts failin a sequence with H3 being the last host to fail.After H1 and H2 recover, the cluster has a quorum (one host failure tolerated). Despite this, vSphere HA isunable to restart the virtual machine because the last host that failed (H3) contains the most recent copy ofthe virtual machine object and is still unaccessible.In this example, either all three hosts must recover at the same time, or the two-host quorum must includeH3. If neither condition is met, HA attempts to restart the virtual machine when host H3 comes back on line.VMware, Inc. 179