Why ESX entering into Maintenance Mode

1. Why to place a Host in Maintenance Mode

You place a host in maintenance mode when you need to service it, for example, to upgrade memory, CPU OR to install an upgrade, etc. A host enters or leaves maintenance mode only as the result of an admin/user’s request.

• Virtual machines that are running on a host that is entering maintenance mode needs to be migrated to another host (either manually or automatically by DRS) or it is shut down. 
• The host is in a state of Entering Maintenance Mode until all running virtual machines are powered down or migrated to different hosts. 
• You cannot power on virtual machines or transfer (migrate)other virtual machines to a host entering maintenance mode. 
• When no more running virtual machines are on the host, the host’s icon changes to include “Maintenance Mode” and the host’s Summary panel (left side) indicates the new state. 
• While in maintenance mode, the host does not allow you to deploy or power on a virtual machine. 
• Once the required work is completed, the ESX host is relieved from “Maintenance Mode” using Exit Maintenance Mode option. 
• Once the ESX host comes out of maintenance mode, you can deploy,vMotionOR power on a virtual machine. 
• If HA / DRS is enabled in fully automatic mode then vCenter will automatically rationalise the load between ESX hosts in the cluster & move VMs on/off the ESX host

2. Procedure to put ESX in Maintenance Mode 

• Login to vCenter client using user/admin credentials. 
• Once logged in, using summary panel on the left side, expand to find the required ESX server which needs to be put in Maintenance Mode. 
• Once you select “Enter Maintenance Mode”, check if the task has been initiated / completed at the bottom pane of the VCenter client. 
• For a host to enter maintenance mode all powered-on virtual machines must be migrated off that ESX host. When you request that a host enter maintenance mode, you are also asked whether you want to migrate all the powered-off virtual machines on that host to other hosts in the cluster. 
• Note, you may see some windows asking for confirmation. Select YES&OK. 
• Check the status at Recent tasks window if the activity has started & monitor the progress. 
• If the activity is completed in the “recent task” window then check if “Maintenance mode” is now displayed against the concerned ESX host.

3. Issues - VMs do not vMotion when attempting to enter maintenance mode. 

If you have HA configured it should automatically vmotion off the VMs to another ESX host with enough resources. 

If you are also using DRS it will do some load balancing and also work out any affinity rules. Maintenance mode can get stuck at 2% progress when there are still active VMs running on that ESX host. 

An ESX Server will not complete going into maintenance mode until there are no running VMs running on it. 

If HA is configured it should automatically vMotion off the VMs to another ESX host with enough resources. 

If DRS is also enabled then it will do some load balancing and also work out any affinity rules. 

If the “Enter maintenance mode” task is struck for long time then some of the known issues are listed below. 

• Usage of local storage instead of shared storage. 
• Some of the VMs were not migrated off this ESX host to another. 
• VMware HA/DRS cluster is not enabled. 
• The setting is not set to “Fully Automated” mode.

Possible solutions are:- 

• If you are using local storage instead of shared storage then use vCenterWebclient to move the VMs off the ESX & then put the host in Maintenance Mode. 
• If some of the VMs were not migrated off the concerned ESX host then shutdown it. 
• Enable VMware HA/DRS cluster. 
• Set the DRS Automation Level to “Fully Automated” mode