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tscooter
Contributor
Contributor

Calculate Slots

Hi,

At the moment our slots situation looks like this:

VMware HA slot size: 256 MHz, 4 vCPU, 221 MB RAM.

HA Advanced Runtime Info: Total slots 183, used slots 23, available slots 99.

Now when I try to reserve 4GB of RAM to a VM it fails to start because it exceeds the HA failover capacity. ( Insufficient resources to satisfy configured failover level for HA).

So I would like to configure the slot size so that all VM's without reserved memory with low values like the above (256 MHz, 4 vCPU, 221 MB RAM) and those VM's with reserved memory should should always get the memory they have reserved. Is this possible so that I can reserve 4 GB to a VM?

I'm unable to set das.slotMemInMBto 4096 because then we would only have the HA resources for 10 VM's Smiley Happy

Thomas

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depping
Leadership
Leadership

I think you are mixing up a couple of things.

First, when you set a reservation of 4GB the default slot size for memory will become 4GB. This means that you will have a lower amount of slots as the slots are larger.

If you want or need to use reservations it is probably better, when you are on vSphere, to use a Percentage Based admission control policy instead of a number of host failures. This actually just adds up all the reservations, or defaults, and subtracts it from the amount of available resources. In other words, no slotsizes -> increased flexibility.

Of course if you still want to use the number of host failures instead of a percentage you can just set a reservation and set "das.slotMemInMB" low. This creates a fixed slotsize for memory which will be lower than any of your reservations. Your reservation on a VM will still work and be used, will also work for DRS admission control but not for HA admission control.

Is that clear enough?

For more detailed info I would recommend reading my deepdive on HA:

http://www.yellow-bricks.com/vmware-high-availability-deepdiv/



Duncan

VMware Communities User Moderator | VCDX

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tscooter
Contributor
Contributor

Thanks for your reply, I have now read the article you suggested and things are more clear.

Yes, when I reserve 4GB to a VM it will set the default slot size for memory to 4GB and since this means I can only have 10 VM (10 free slots) it fails to power on the VM that has 4GB reserved because at the moment we have 27 VM's running.

So my only options are to disable admission control (don't want to do that) so this leaves me to the only option "Precentage Based admission"?

It's a little bit unclear to me how this policy work? I can see that the default value is 25% is that a good number? We have 3 hosts with the same amount of memory and cpu. If one goes down, what happens to the VM's on that host and if 2 goes down what happens then? At the moment we have 27 VM's and only 2 of them will have 4 or 6 GB of reserved RAM (Total cluster amount of RAM 130GB).

Also, can I change the policy on a running cluster with running vm's, will there be any impact?

Thomas

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nava_thulasi39

Thanks.

It gives a very clear idea about HA.

If you find this or any other answer useful please consider awarding points by marking the answer correct or helpful
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ThompsG
Virtuoso
Virtuoso

Hi,

So my only options are to disable admission control (don't want to do  that) so this leaves me to the only option "Precentage Based admission"?

Not quite true. As depping has said you can use the das.slotMemInMB advanced option to specify an upper boundary for your slot size and therefore still use the Host failures option, say something like 1024MB.

And to answer your other question, yes you can make these changes on a running cluster.

Kind regards,

Glen

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depping
Leadership
Leadership

You can make all the changes for HA on a running cluster. you could use the percentage based admission control policy, how it works is described in the same article. but simply said it will subtract all reserved (or the minimal) resource from the available resources and unless the threshold is reached it will allow you to power on VMs

You could also use das.slotMemInMB but I don't see the real value of using it as it defeats the whole purpose of "slots" in the first place.

Duncan (VCDX)

Available now on Amazon: vSphere 4.1 HA and DRS technical deepdive

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admin
Immortal
Immortal

>> You could also use das.slotMemInMB but I don't see the real value of  using it as it defeats the whole purpose of "slots" in the first place.

Well, that's not quite true Smiley Happy. The "slot policy" has clear disadvantages in some scenarios, but one of it's big advantages is that it provides stronger guarantees of failover because it considers resource fragmentation which the % policy does not. Admittedly, this is less of an advantage in 4.1 since we added the ability of HA to tell DRS to defragment the cluster if there are not enough resources for failover (of course, that requires DRS to be enabled and VC to actually be running; also defragmentation may not always be possible in some corner cases). Even if you use the das.slotMemInMB advanced option, you still have a higher guarantee of failover than the % policy assuming that most vms fit into 1 slot (the risk of fragmentation is only on the large vms that require multiple slots).

Elisha

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depping
Leadership
Leadership

I understand Elisha, I know how it works... but I am merely trying to make a point. If you set the slotsize low because of a high reservation (what the original question is) chances are you still run into an issue where resources are fragmented as the VM that has the large reservation will take multiple slots and HA is not aware of that. Chances of that are similar to the % based policy.

Duncan (VCDX)

Available now on Amazon: vSphere 4.1 HA and DRS technical deepdive

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