The effect of reserved memory on HA calculations depends on which of these admission control policies were selected:
Host Failures Cluster Tolerates
Under this policy, HA uses slot sizes to calculate the number of VMs that can fail over based on the specified number of host failures. Slot sizes are calculated by means of reservations on the VMs. If there are no reservations on the VMs, then the following defaults are used:
- CPU: 32MHz
- Memory: 0MB + memory overhead
HA slots with no memory reservation on a VM:
HA slots with 2048MB memory reservation on a VM:
The number of HA slots available in the cluster is affected because the DSVA has reserved memory. You may consider using the HA advance attribute das.slotmeminmb to override the memory slot size.
Percentage of Cluster Resources Reserved
Under this policy, the total amount of reservations in the cluster is considered when calculating the available resources. The basic formula used is as follows:
(Total Cluster Resources - Total Reservations) / Total Cluster Resources = ____ %
As an example, if a cluster has 100GB of memory and a total of 20GB in memory reservation, there will be 80GB of memory available in the cluster to start the VMs. From this number, a certain percentage will still need to be allocated for failover.
The following example shows how memory reservation affects the total resources available. A cluster with no memory reservation has 98% Current Memory Failover Capacity:
By adding reservations of 2048MB on two (2) VMs in the cluster (thus, a total of 4096MB), the Current Memory Failover Capacity is reduced to 89%:
Specify Failover Hosts
Under Specify Failover Hosts Policy, the DSVA memory reservation does not affect the HA calculations in the cluster.
Note: It is important to understand that DSVA does not fail over to another ESXi host, but affects the HA calculations