VSAN-Troubleshooting-Reference-Manual

Diagnostics and Troubleshooting Reference Manual – Virtual SAN
Maintenance mode and 3 node clusters
Putting one node of a 3-node cluster into maintenance mode with full data
migration will show the selected host "Enter Maintenance Mode" stuck at 2%. This
is expected behavior.
If there is a 3-node cluster, and all hosts are providing storage, this will meet the
NumberOfFailuresToTolerate=1 policy, as the formula for the number of hosts
required to provide storage is 2n+1 where "n" is the number of FTT. In order to
satisfy the FTT=1 there needs to be three hosts providing storage.
If one host in 3-node cluster is placed into maintenance mode, this leaves two hosts
in the cluster providing storage. Now keep in mind that components and witnesses
cannot reside on the same host as the following example is examined:
Host #1 has a disk component mirror for VM1
Host #2: has a second disk component mirror for VM1
Host #3: has a witness for disk object for VM1
If host 1 is placed into maintenance mode with the option "Full Data Migration"
chosen, Virtual SAN will attempt to make a new replica mirror. Unfortunately there
are not enough hosts in the cluster to accommodate this request, so the task halts as
described.
If there was a 4th host in the cluster also providing storage, this maintenance mode
would be successful as the new mirror component could be created on host 4.
Choosing "Ensure Accessibility" would be the only option in this case, because
Virtual SAN will first of all check there is an online copy of the data somewhere else
within the cluster, even though the VM Disk will report as "Out of Compliance"
whilst that single host is in maintenance mode, without a 4th host, this is the correct
choice.
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