3/28/2022

Vmware Cluster Slot Size Calculation

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  1. The slot has two parts, the CPU component and the memory component. Each of them has its own calculation. If there are no virtual machine resource reservations in the cluster, then the slot size (for ESXi 5 at least) is 32 Mhz for CPU and 0 MBs + overhead for memory. (I’ve used 80 MBs as my memory overhead in the examples).
  2. One of the things you have to do when building out your VMware cluster is size the amount of memory (RAM) you need. From what I have seen most people assume that by adding up the total amount of RAM they have on their physical servers they will get the total amount of memory that is required for their cluster; those people are wrong.
  3. The cluster has two host nodes, each with 24 GB of RAM and 2 Quad Core Procs. Host reserves are all on the defaults. So 512 KB of RAM is reserved for the hosts and 20 percent CPU.

VMWare vShpere Slot Calculation

What is SLOT?
As per VMWare's Definition,
'A slot is a logical representation of the memory and CPU resources that satisfy the requirements for any powered-on virtual machine in the cluster.'
If you have configured reservations at VM level, It influence the HA slot calculation. Highest memory reservation and highest CPU reservation of the VM in your cluster determines the slot size for the cluster.
Here is the Example,
If you have the VM configured with the highest memory reservation of 8192 MB (8 GB) and highest CPU reservation of 4096 MHZ. among the other VM's in the cluster, then the slot size for memory is 8192 MB and slot size for CPU is 4096 MHZ. in the cluster.
If no VM level reservation is configured , Minimum CPU size of 256 MHZ and memory size of 0 MB + VM memory overhead will be considered as CPU and Memory slot size.
Calculation for Number of Slots in cluster :-
Once we got the Slot size for memory and CPU by the above method , Use the below calculation
Num of CPU Slots = Total available CPU resource of ESX or cluster / CPU Slot Size
Num of memory slots = Total available memory resource of ESX or cluster minus memory used for service console & ESX system / Memory Slot size
Let's take a Example,
I have 3 host on the cluster and 6 Virtual machine is running on the cluster and Each host capacity as follows
RAM = 50 GB per Host
CPU = 8 X 2.666 GHZ per host
Cluster RAM Resources = 50 X 3 = 150 GB - Memory for service console and system = 143 GB
Cluster CPU resources = 8 X 2.6 X 3 = 63 GHZ (63000 MHZ) of total CPU capacity in the cluster - CPU Capacity used by the ESX System = 60384 MHZ



I don't have any memory or CPU reservation in my cluster, So, the default CPU slot size 256 MHZ and one of my Virtual machine is assigned with 8 vcpu and its memory overhead is 344.98 MB (which is the highest overhead among my 6 virtual machines in the cluster)
Let's calculate the num of CPU & Memory slots
Num of CPU Slots = Total available CPU resource of cluster / CPUSlot size in MHZ
No of CPU Slots = 60384 MHZ / 256 MHZ = 235.875 Approx
Num of Memory Slots = Total available Memory resource of cluster / memory Slot Size in MB
Num of Memory Slots = 146432 / 345 = 424 Approx
The most restrictive number among CPU and Memory slots determines the amount of slots for this cluster. We have 235 slots available for CPU and 424 Slots available for Memory. So the most restrictive number is 235.
So, Total number of slots for my cluster is 235 Approx. Please find the below snapshot

Please refer my blog post on Understanding Total Slots, Used Slots & Available slots in VMware HA Slot to understand Total Slots, Used Slots & Available slots
I hope you understand the post..Thanks For Reading !!!!!

How does it calculate and our mind will think of the below calculation Available slots = Total slot – Used slots i,e Available slots = 234 – 6 = 228. It should come as 228 as available slots but why Available slots is 150 in the above snapshot. Is that wrong or VMware did something wrong in HA slot calculation? If no VM level reservation is configured, Minimum CPU size of 256 MHZ and memory size of 0 MB + VM memory overhead will be considered as CPU and Memory slot size. Calculation for Number of Slots in cluster:-Once we got the Slot size for memory and CPU by the above method, Use the below calculation.

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.

Slot

Using the slot policy, vSphere HA performs admission control in the following way:

Vmware Cluster Slot Size Calculations

  1. Calculates the slot size.

    A slot is a logical representation of memory and CPU resources. By default, it is sized to satisfy the requirements for any powered-on virtual machine in the cluster.

  2. Determines how many slots each host in the cluster can hold.
  3. Determines the Current Failover Capacity of the cluster.

    This is the number of hosts that can fail and still leave enough slots to satisfy all of the powered-on virtual machines.

  4. Determines whether the Current Failover Capacity is less than the Configured Failover Capacity (provided by the user).

    If it is, admission control disallows the operation.

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.

Vmware Cluster Slot Size Calculation Calculator

  • vSphere HA calculates the CPU component by obtaining the CPU reservation of each powered-on virtual machine and selecting the largest value. If you have not specified a CPU reservation for a virtual machine, it is assigned a default value of 32MHz. You can change this value by using the das.vmcpuminmhz advanced option.)
  • vSphere HA calculates the memory component by obtaining the memory reservation, plus memory overhead, of each powered-on virtual machine and selecting the largest value. There is no default value for the memory reservation.

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.

Vmware Cluster Slot Size Calculation

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.

Vmware Cluster Slot Size Calculation Tool

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.

Calculation

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:

Vmware Cluster Slot Size Calculation Chart

  • The cluster is comprised of three hosts, each with a different amount of available CPU and memory resources. The first host (H1) has 9GHz of available CPU resources and 9GB of available memory, while Host 2 (H2) has 9GHz and 6GB and Host 3 (H3) has 6GHz and 6GB.
  • There are five powered-on virtual machines in the cluster with differing CPU and memory requirements. VM1 needs 2GHz of CPU resources and 1GB of memory, while VM2 needs 2GHz and 1GB, VM3 needs 1GHz and 2GB, VM4 needs 1GHz and 1GB, and VM5 needs 1GHz and 1GB.
  • The Host Failures Cluster Tolerates is set to one.
  1. Slot size is calculated by comparing both the CPU and memory requirements of the virtual machines and selecting the largest.

    The largest CPU requirement (shared by VM1 and VM2) is 2GHz, while the largest memory requirement (for VM3) is 2GB. Based on this, the slot size is 2GHz CPU and 2GB memory.

  2. Maximum number of slots that each host can support is determined.

    H1 can support four slots. H2 can support three slots (which is the smaller of 9GHz/2GHz and 6GB/2GB) and H3 can also support three slots.

  3. Current Failover Capacity is computed.

    The largest host is H1 and if it fails, six slots remain in the cluster, which is sufficient for all five of the powered-on virtual machines. If both H1 and H2 fail, only three slots remain, which is insufficient. Therefore, the Current Failover Capacity is one.

Vmware Cluster Slot Size Calculation Calculator

The cluster has one available slot (the six slots on H2 and H3 minus the five used slots).