• The behavior described in this article applies to VirtualCenter 2.5 Update 3 and later.
• VMware HA (High Availability) strict admission control is enabled by default. You can disable it in the VI Client by opening the VMware HA section of the cluster's Settings dialog box and selecting Allow VMs to be powered on even if they violate availability constraints check box.
• When VMware HA is used with Distributed Resource Scheduler (DRS) or VMwareDistributedPowerManagement (DPM), you should be aware of the possible implications of enabling or disabling VMware HA strict admission control.
• When using strict admission control, under certain conditions, you may receive the error:
  
  Insufficient Resources for HA failover

VMware vCenter Server 5.0.x
VMware vCenter Server 5.1.x
VMware vCenter Server 4.1.x
VMware vSphere ESXi 5.1
VMware ESX 4.0.x
VMware ESX Server 3.5.x
VMware vSphere ESXi 6.0
VMware VirtualCenter 2.5.x
VMware vSphere ESXi 5.0
VMware ESXi 3.5.x Embedded
VMware ESXi 4.1.x Installable
VMware ESX 4.1.x
VMware ESXi 3.5.x Installable
VMware vCenter Server 6.0.x
VMware ESXi 4.0.x Embedded
VMware vCenter Server 4.0.x
VMware ESXi 4.0.x Installable

Implications of enabling VMware HA strict admission control

You cannot power on the virtual machine when using VMware HA strict admission control .Trying to power on the virtual machine would violate availability constraints. (in other words, cause the current failover level to become less than the configured failover level). When such an attempt is made, a message informs you that the virtual machine cannot be powered on.

When VMware HA strict admission control is enabled, DRS and VMware DPM protect the availability of failover capacity at all times, and only take actions or make recommendations that are consistent with ensuring the availability of that capacity.

• DRS does not evacuate virtual machines from a host for the purpose of placing it in maintenance mode or standby mode if placing the host in this state would violate failover requirements. You can still manually evacuate virtual machines in order to place hosts in maintenance mode or standby mode. If you violate failover requirements by doing this, however, the cluster turns red.
• VMware DPM does not place hosts in standby mode if doing so would violate failover requirements.
• If necessary, VMware DPM powers on hosts (brings them out of standby mode) to maintain failover needs.
• A minimum of two hosts must be powered on in any HA-enabled cluster.

This behavior, while providing the most secure failover protection for your cluster, might create issues in certain scenarios:

· If you violate the failover constraints because there is a temporary, but nontrivial, time period in which there are not enough resources to support them. For example, if you are placing hosts in standby mode to test them for use with DPM.

· If any automated processes take actions that violate the failover constraints. For example, as part of an upgrade directed by VMware Update Manager.

If you encounter such issues, you must disable strict admission control.

Implications of disabling VMware HA strict admission control

When you disable VMware HA strict admission control, virtual machines can be powered on even if there are not sufficient resources to ensure failover capacity. When this is done, no warnings are presented, and the cluster does not turn red.

When strict admission control is disabled, VMware HA failover resource constraints are not passed on to DRS and VMware DPM. Thus, the constraints are not enforced:

• DRS does evacuate virtual machines from hosts and place the hosts in maintenance mode or standby mode regardless of the impact this might have on failover requirements.
• VMware DPM does power off hosts (place them in standby mode) even if doing so violates failover requirements.

Your particular usage scenario might make disabling VMware HA strict admission control desirable. You should be aware, that doing so (when interoperating with DRS and VMware DPM or not) can lead to a reduced level of availability and failover protection for your cluster.

Additional Information

For translated versions of this article, see:

• 日本語: DRS および VMware DPM の使用時に VMware HA の厳密なアドミッション コントロールを有効または無効にする場合の影響 (2081795)
  
• 简体中文: 使用 DRS 和 VMware DPM 时启用或禁用 VMware HA 严格接入控制的影响 (2087024)