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Danny1975
Contributor
Contributor

HA issue

I'm in a bit of a debate with my colleagues regarding HA issues we're experiencing.

to me the issue seems rather cut and dry, but i can't get my point across well enough.

a quick overview of our environment:

4 node ESX 3.5 cluster with DRS and HA.

each node has 2 quad core 2.7GHz CPU's and 32 GB mem.

there are 81 active VM's on the system.

13 VM's have 4 CPU's assigned to them

22 VM's have 2 CPU's assigned to them

the rest has 1 CPU assigend.

there are some limits set, but no reservations (none for CPU, none for memory).

we're now encountering the problem that we cannot add new VM's, as that would violate HA constraints.

my point is that this is likely caused by the lack of configured slots, since no reservations are set, the default is chosen, which is 256MHz per CPU as far as i know. HA, if not configured, will create slots based on the worst case scenario for CPU and mem, disregarding memory for the moment, for our setup that would mean a slot size of 1GHz. if the slot is 1GHz and the total CPU available is 83GHz, then 83 VM's can be deployed, new machines will result in HA constraints being violated.

my suggestion was to create a custom slot size of 512 MHz, which should result in at least 50-60 more VM's to be able to work on this cluster.

am i correct in my thoughts, or am i way off the mark here?

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mark24
Enthusiast
Enthusiast

VMware HA slot calculation is done by the vCenter HA service and provides the capacity of the cluster as a whole to the various agents involved.

In VirtualCenter 2.x, the virtual machine with the maximum resource consumption was the one chosen as the basis of the slot calculation. This posed a problem if there was only one heavily resourced virtual machine and other virtual machines did not consume as many resources.

In vCenter Server 4.0, the slot size is now shown in vSphere Client on the Summary tab of the cluster.

VMware HA determines how many slots are available in each ESX/ESXi host based on the host’s CPU and memory capacity. It then determines how many ESX/ESXi hosts can fail in the cluster with at least as many slots as powered on virtual machines. If a virtual machine does not have reservations, meaning that the reservation is 0, default values of 0 MB of RAM and 256 MHz CPU speed are used.

Here is an HP article that goes into detail about calculating HA slot capacity: http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?lang=en&cc=us&taskId=120&prodSeriesId=...

Hope this helps

Best regards,

Mark Ramos VCP 3&4

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Best regards,
Mark Ramos VCP 3&4
Danny1975
Contributor
Contributor

that indeed helps, it confirms my thoughts in regards to our current slot size.

i wasn't aware that the default mem size is set to 0 if no reservations where set, i thought the default was dependent on the VM using the most memory.

so i guess it's safe to assume our slot size is 1GHz with 0 MB memory, for our setup that results in 83VM's. where i to set a slot size of 512MHz/0Mb, i would effectively double the number of slots. The 4 CPU VM's i have would simply take up 2 slots, but there would still be around 60 slots to fill after this change, that's 60 more then we have now.

i was also contemplating going for 256MHz slots. there's a risk involved of course, but it would greatly improve efficiency. with our current setup, our hosts are hardly using their CPU.

in any case, thanks for the reply, most helpfull.

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