Vmware Cluster Slot Size Calculation
Seems like the formula ‘Available slots’ = ‘Total slots in cluster’ – ‘Used slots’does not apply. The values I see are ‘Total slots in cluster’ = 36 ‘Used slots’ = 7 ‘Available slots’ = 11. Please note no advanced parameters have been used to alter the slot size values.Can send you a screenshot if required.
VMWare vShpere Slot Calculation
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 !!!!!
A High Availability (HA) slot size calculation becomes distorted when using VMware strict admission control The cluster summary displays a message ' Insufficient configured resources to satisfy the desired vSphere HA failover level on the cluster '. Esx252 - 8.1.994 cpu and 32762.87MB Memory If I go to run time info of cluster in vmware I can it has used 32MHz 1, Virtual CPU and 40MB memory slot size. Then it says total slot is 691 and failover slot is 422. Jun 28, 2017 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?
We get an enormous amount of questions about VMware’s HA (High Availability), especially when users see a message stating there are Insufficient resources to satisfy HA failover. We have already discussed the mechanism that HA uses to provide high availability here. Now we need to understand capacity calculations. In current versions of ESX (3.02) and earlier the following calculation applies for failover capacity.
HA Failover Capacity
Failover Capacity is determined using a slot size value that is calculated on the cluster. Slots are calculated by a combination of the total CPU and Memory that are in the physical hosts. The calculation for failover capacity works as follows:
Let’s say you have 4 ESX servers in your VMware HA cluster and Configured Failover capacity on the cluster is set to 1.
Physical memory in the hosts is as follows:
ESX1 = 16 GB
ESX2 = 24 GB
ESX3 = 32 GB
ESX4 = 32 GB
Vmware Cluster Slot Size Calculation Tool
In the cluster, you have 24 VM’s each configured and running. Of the 24 VM’s running, determine the VM which has the highest “configured memory”. For this example let’s say this is 2GB. All other VMs are configured with less or equal to 2GB.
With this information we can now do the calculation:
1. Pick the ESX host which has the least amount of RAM. In this case, it is ESX1 and the minimum amount of RAM is = 16 GB
2. Divide the value found in step 1 with value for the maximum RAM in a VM. In my example, this gives us 8 (16 divided by 2). This means we have 8 slots available per ESX host in the cluster.
Vmware Cluster Slot Size Calculation Calculator
3. Since we have 4 hosts and the configured failover capacity for the cluster is 1, we are left with 3 hosts in a failure situation. Hence the total number of VMs that can be powered on these 3 servers is 24 VMs. (i.e. 8 multiplied by 3 = 24)
4. If the total number of VMs in the cluster exceeds 24 then it will give us “Insufficient resources to satisfy HA failover” and the “current failover capacity will be shown as 0”. If the number is less than 24, we should not get this message.
Note: If you are still seeing the message and you have less VM’s running than in the calculation allows for, check both the CPU and Memory reservations on both VM’s and resource pools, as this can skew the calculation. You should avoid unnecessary memory or CPU reservations on VM’s as this can cause these types of errors to occur because we have to ensure that the resource is available.
There are multiple ways to fix or get around this calculation. The most common are as follows:
• Set the “Allow Virtual Machines to be powered on even if they violate availability constraints” in the configuration of the cluster. In this case, it ignores the above calculation and will try to power on as many VM’s as possible in case of HA failover. If this is the option chosen you can also set restart priority in the ‘Virtual Machine Options’ section of the cluster configuration. This way any high priority VM’s are powered on first, and then the lower priority up to the point where we cannot power any further VM’s on
• If you have one VM which is configured with a very high amount of memory, you can either lower its configured memory or take it out of the cluster and run it on any other standalone ESX host. This will increase the number of slots available with the current hardware
• Increase the amount of RAM on servers so that there are more slots available with the current RAM reservations.
• Remove any CPU reservations on any VM(s) that are greater than the max speed of the processors in the hosts. For example, if the CPU Usage on the summary tab of your ESX Server shows as follows:
Then you will see the error message popup if you have a CPU reservation greater than 2793MHz on a VM.
Note: The above calculation method is very limited and is going to be revised in future releases of VirtualCenter to improve calculations for HA failover.