Entering Maintenance Mode but VM won’t be automatically migrated to other ESX Host by DRS
Then I found this thread on VMTN which explained everything.
This is not a BUG. However, is the way we understand the HA Settings and Functionality. When in “HA Settings” you specify the “Failover Capacity” as “1″ and you have a 2 NODE Cluster, you are simply telling the HA that in any given instance it will have “At least 1″ spare HOST. Now, when you Manually or Using Update Manager try to put a HOST in Maintenance Mode, HA Failover Capacity is “Violated” because while the HOST is in Maintenance Mode, there is “NO Spare HOST” for HA. Meaning in an event the Second Node goes down, everything goes down and HA will never work. This a Straight Violation to the “Failover Capacity” that you have specified.
Hence, by all means in a 2 NODE Cluster you have to “Allow VMs to Power On even if they violate the availability constrains” if you want them to be Automatically Migrated when you put HOST on Maintenance Mode or use Update Manager. If you don’t want to change this setting and still use this feature you need to add another HOST to the Cluster while keeping the Failover Capacity at 1.
