With the slot policy option, vSphere HA admission control ensures that a specified number of hosts can fail and sufficient resources remain in the cluster to fail over all the virtual machines from those hosts.
Using the slot policy, vSphere HA performs admission control in the following way:
You can expect at least 1 or 2 questions on your 5.1 and 5.5 certifications regarding VSA. NFS is presented on the network by the VSA. The vSphere VSA is used to aggregate unused local storage on ESXi hosts and present that storage as shared storage on the network.
Note:
You can set a specific slot size for both CPU and memory in the admission control section of the vSphere HA settings in the vSphere Web Client.
Slot Size Calculation
Slot size is comprised of two components, CPU and memory.
If your cluster contains any virtual machines that have much larger reservations than the others, they will distort slot size calculation. To avoid this, you can specify an upper bound for the CPU or memory component of the slot size by using the das.slotcpuinmhz or das.slotmeminmb advanced options, respectively. See vSphere HA Advanced Options.
You can also determine the risk of resource fragmentation in your cluster by viewing the number of virtual machines that require multiple slots. This can be calculated in the admission control section of the vSphere HA settings in the vSphere Web Client. Virtual machines might require multiple slots if you have specified a fixed slot size or a maximum slot size using advanced options.
Using Slots to Compute the Current Failover Capacity
After the slot size is calculated, vSphere HA determines each host's CPU and memory resources that are available for virtual machines. These amounts are those contained in the host's root resource pool, not the total physical resources of the host. The resource data for a host that is used by vSphere HA can be found on the host's Summary tab on the vSphere Web Client. If all hosts in your cluster are the same, this data can be obtained by dividing the cluster-level figures by the number of hosts. Resources being used for virtualization purposes are not included. Only hosts that are connected, not in maintenance mode, and that have no vSphere HA errors are considered.
The maximum number of slots that each host can support is then determined. To do this, the host’s CPU resource amount is divided by the CPU component of the slot size and the result is rounded down. The same calculation is made for the host's memory resource amount. These two numbers are compared and the smaller number is the number of slots that the host can support.
The Current Failover Capacity is computed by determining how many hosts (starting from the largest) can fail and still leave enough slots to satisfy the requirements of all powered-on virtual machines.
Admission Control Using Slot Policy
The way that slot size is calculated and used with this admission control policy is shown in an example. Make the following assumptions about a cluster:
The cluster has one available slot (the six slots on H2 and H3 minus the five used slots).
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Slot Policy Admission ControlJul 11, 2012 . “VMware HA slots is a logical representation of the memory and CPU resources that satisfy the requirements for vsphere 5.1 default slot size any powered-on virtual machine in the cluster.” . Highest memory reservation and highest CPU reservation of the VM in your cluster determines the slot size for the cluster.Share this:vmnomad Feb 12, 2016 at 05:35 UTCVMWare HA Slots Calculation - Deep Dive to Understand
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