Posted on March 30, 2015
There is a subtle difference in maintenance mode behaviours between VSAN version 5.5 and VSAN version 6.0. In Virtual SAN version 5.5, when a host is placed into maintenance mode with the “Ensure Accessibility” option, the host is maintenance mode continues to contribute its storage towards the VSAN datastore. In other words, any VMs that had components stored on this host still remained fully compliance with all of the components available. In VSAN 6.0, this behaviour changed. Now, when a host is placed into maintenance mode, it no longer contributes storage to the VSAN datastore, and any components that reside on the physical storage of the host that is placed into maintenance mode is marked as absent. The following screen shots show the behaviour.
1. VM is fully compliant, all components are active:
2. Host is placed into maintenance mode, ensure accessibility is chosen. The host being placed into maintenance mode is cs-ie-h02, which hosts one of the RAID 1 components for the hard disk above:
3. VM hard disk is non-compliant, component is absent:
Why did we change this behaviour? Well, we felt that the previous maintenance mode behavior was misleading. When hosts were in maintenance mode, and VMs were shown as fully compliant, one might think that the hosts could be removed from the VSAN cluster without any sort of impact, which is not the case. In fact, looking at the VM compliance state would lead one to believe that there are no hosts in maintenance mode and that hosts could be removed from the cluster.
This new behaviour draws attention to the fact there are hosts in maintenance mode, and that taking hosts out of the cluster (or indeed a failure in the cluster) could lead to VM unavailability.
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