When no more running virtual machines are on the host, the host’s icon changes to include under maintenance
and the host’s Summary panel indicates the new state. While in maintenance mode, the host does not allow
you to deploy or power on a virtual machine.
NOTE DRS does not recommend (or perform, in fully automated mode) any virtual machine migrations off
of a host entering maintenance or standby mode if the VMware HA failover level would be violated after the
host enters the requested mode.
Using Standby Mode
When a host machine is placed in standby mode, it is powered off.
Normally, hosts are placed in standby mode by the VMware DPM feature to optimize power usage. You can
also place a host in standby mode manually. However, DRS might undo (or recommend undoing) your change
the next time it runs. To force a host to remain off, place it in maintenance mode and power it off.
Removing Virtual Machines from a Cluster
You can remove virtual machines from a cluster.
You can remove a virtual machine from a cluster in two ways:
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When you remove a host from a cluster, all of the powered-off virtual machines that you do not migrate
to other hosts are removed as well. You can remove a host only if it is in maintenance mode or disconnected.
If you remove a host from a DRS cluster, the cluster can become yellow because it is overcommitted.
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You can migrate a virtual machine from a cluster to a standalone host or from a cluster to another cluster
using the Migrate Virtual Machine wizard. To start this wizard either drag the virtual machine object on
top of the cluster object or right-click the virtual machine name and select Migrate.
If the virtual machine is a member of a DRS cluster rules group, vCenter Server displays a warning before
it allows the migration to proceed. The warning indicates that dependent virtual machines are not
migrated automatically. You have to acknowledge the warning before migration can proceed.
DRS Cluster Validity
The vSphere Client indicates whether a DRS cluster is valid, overcommitted (yellow), or invalid (red).
DRS clusters become overcommitted or invalid for several reasons.
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A cluster might become overcommitted if a host fails.
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A cluster becomes invalid if vCenter Server is unavailable and you power on virtual machines using a
vSphere Client connected directly to an ESX/ESXi host.
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A cluster becomes invalid if the user reduces the reservation on a parent resource pool while a virtual
machine is in the process of failing over.
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If changes are made to hosts or virtual machines using a vSphere Client connected to an ESX/ESXi host
while vCenter Server is unavailable, those changes take effect. When vCenter Server becomes available
again, you might find that clusters have turned red or yellow because cluster requirements are no longer
met.
When considering cluster validity scenarios, you should understand these terms.
Reservation
A fixed, guaranteed allocation for the resource pool input by the user.
Reservation Used
The sum of the reservation or reservation used (whichever is larger) for each
child resource pool, added recursively.
Unreserved
This nonnegative number differs according to resource pool type.
vSphere Resource Management Guide
56 VMware, Inc.
