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fwendel
Contributor
Contributor

HA Slots size -Only 12 slots and admision control disable

Hello

I have a customer with an issue with the slot size. They have implemented cpu and memory reservation, and after that the total slot size in the cluster are 12.

The cluster has 3 ESXi 5.1 with the same amount of memory and cpus.

There some vms with: 32768MB of reservation and 19155MHZ of cpu reservation.

My question are: Why only 12 slots in the cluster? How Vmware calculate 12 slots in total? In case of a hardware failure, can the cluster power on all the vms in the remaining 2 servers?

Regards

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JPM300
Commander
Commander

Hey fwenclel,

Slot sizes are always tricky and typically require a fair amount of baby sitting once you start getting into reservations as reservations start screwing with the slot sizes.  You can still manually configure the slot size to fix these issues however when you create manual slot sizes you run into slot fragmentation issues.  A good read on this is here: http://www.yellow-bricks.com/vmware-high-availability-deepdiv/#HA-admission-howdoeshacalculate

This blod is 4.1 however the concepts still apply in 5.x.  With that said the easiest way to make sure your clients VM's will start is to use the % instead of slot size when it comes to admission control.  This way you don't have to baby sit the slot size and the reserveations don't have as much of an impact on the cluster.

In its current configuration, your VM's will not start in the event of a failure as it is showing you only have 12 slots per server with 121 VM's running which will requires slots as well.  Like I was saying before using the % admission control is typically a MUCH easier way of assuring your VM's will start and ease on configuring your cluster as % are easy to work with opposed to figuring out the slot size everytime something changes in the evnviroment.

To awenser your queastion.  Your slot size is 12 becuase the HIGH CPU reservation / memory reservation is pushing the slot size out for everything as it takes the worse case scenario.  Even if you didn't have reservations the slot size will shift to whatever the biggest VM is. So say for instance you have 10 vm's all 1vCPU 1GB of memory, but you have 1 VM that is 1vCPU and 32GB of memory.  Your memory slot size will shift to 32GB as it has to appease the size of that VM in the event of a failure and the only way to do this automatically is to shift the slot size to assure it can get what it needs.

I hope this has helped.

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fwendel
Contributor
Contributor

Hello

thanks for your answer!!!

if the vm does not have reservation,  vmware will use 0mb + overhead to calculate memory slots.

Per my excel each host has 12 slots, why vmware shows only 12 in total?

regards

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JPM300
Commander
Commander

What happens if you take off the reservations temporarily.  What does the slot size change 2?  It should shift immedietly after you remove the reservations.

If you have no vCPU reservations and no memory reservations the slot sizes will use the highest overhead to calculate the slot since, which with no reserveatinos will be the vCPU memory overhead per vm.  So if you have a VM with 4vCPU the memory overhead for vCPU is higher then 1vCPU so it will set it to that.  It still not be nearly as high as to what your slot size is now.  The reason why your cluster is seeing only 12 slots is becuase of the large reservation you have on that 1 VM.  If you remove that you will see your slot size jump up.

Or you can set your adminssion control to %

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Titanomachia
Enthusiast
Enthusiast

if you want more slots despite having large reservations, you will need to edit das.slotCpuInMHz and das.slotMemInMB to specify the biggest slot size you want to calculate against.

Of course there is the risk of not being able to power on a server due to resources in a HA scenario.

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