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java_cat33
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Max VM's per cluster - resource allocation

Hi

I prefer to manage the cluster capacity myself in regards to max VM's per cluster with the configured failover capacity instead of relying on the HA algorithm with VC 2.5

I have a cluster that consists of 3 hosts, each host has identical specs - 22GB memory, 12 Ghz CPU, combined cluster total of 66GB memory and 36Ghz CPU

None of the VM's have reservations, due to this the assumed slot size is 256MB memory/256 Mhz CPU.

The cluster is configured with a failover capacity of 1 host.

The amount of resource in use within the cluster on average is 20GB memory, however the total amount of memory actually assigned to VM's (whether they use it or not) is 35GB.

My question is this - Given that the cluster is configured with a failover capacity of 1 host, theoritically I can assign up to approximately an additional 8GB of memory, to take the amount of alloicated memory to approximately 44GB (I understand there is virtualization overhead, host requirements etc).

Is this correct?

There's currently only 30 VM's in this cluster, so if I was to let VC do the slot size calculation based upon the default of 256MB/Mhz based upon the fact none of the VM's have reservations..... it would be unrealistic (as some VM's have 1GB memory, others 2GB, others 4GB etc)

Thanks in advance.

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kjb007
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Yes, as far as capacity planning goes, we are definitely on the same page, and allocating the additional memory should not be a problem. Transparent page sharing is on by default, and the more you use templates, the better your sharing will get.

I just wanted to clear up the slot calculation thing, as it can cause your cluster to go red, even though you know there are enough resources available to service the vm's you have running. Personally, I've set my slot size to 150 as it was in 3.0.2, and I do my capacity planning myself, and don't rely on slot calculation to tell me how much capacity I have available to me. I guess it's the control factor, but there's only so much I like giving up to automation, and still like to do some things on my own.

Anyway, you are good to go.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB

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java_cat33
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I've just confirmed the below with our reseller - an additional 8GB can be assigned.

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kjb007
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The slot size is a calculation of resources in use, and not assigned. You are doing calculations on memory assigned, so you should be well within the failover capacity for HA. HA will automatically figure out the slot size, and as you stated, if no reservations are configured, then a 256MB slot size will be used for that calculation. You can also change this number in the HA config advanced settings. Open ha settings, advanced, and add das.vmMemoryMinMB and das.vmCpuMinMhz with values less than 256.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
java_cat33
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The slot size is a calculation of resources in use, and not assigned. You are doing calculations on memory assigned, so you should be well within the failover capacity for HA.

The calculation I performed was based upon memory assigned to running VM's - is that incorrect?

If so, I was unaware of that.

Do you plan the amount of VM's you can run in a cluster differently?

Message was edited by: java_cat33

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kjb007
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HA slot size is actually calculated using the amount of memory reservations and cpu reservations. Meaning, if you reserve, say 512mb for a vm, but actually allocate 4 GB to it. The slot size for this vm will be 512 MB. Now, you take the largest memory reservations that you have anywhere in your cluster, and that becomes your default slot. So, if you have 1 vm, with 4 GB reserved, your slot size if 4 GB. So, if you take 4 GB, and take into account your cluster with 66 gb of ram available - capacity = 44 gb gives you 11 vm's before your cluster will complain about having insufficient resources.

Now, if you don't have any reservations at all, then that is where the 256MB number kicks in and that is used instead to calculate how many vm's you can have without going into the insufficient resources for failover status.

The default in the 3.0.2 was around 150 MB, and you can change it back to 150 using the config variables I posted earlier.

Again, this is really for when you want to see the real power of ESX, which is shared memory, and overcommittment. That is where ESX stands head and shoulders above any other virtualization product.

PS, I don't use overcommittment, as I haven't had the need to. Even though I've seen that I typically will not see more than 50-60% memory utilization of what is actually granted. I do my planning similar to yours, but know that doing it that way gives me enough room left over to have resources for host failure.

Hope that clears things a bit,

-KjB

Message was edited by: kjb007 : added PS

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
java_cat33
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Thanks - I appreciate the time you take to respond to these quite lengthy posts :smileygrin:

I am still confused with what you mentioned in an earlier post.... "The slot size is a calculation of resources in use, and not assigned. You are doing calculations on memory assigned". My understanding of slot size is exactly what I first posted and what you have just posted....

I'm glad we're on the same page in regards to cluster capacity planning.... so in my current situation (currently 40GB memory unreserved), allocating approximately another 15GB of memory to new/existing VM's shouldn't be a problem? And as you said... will leave enough resources free if an HA event ever occurs.

Transparent page sharing is used all the time though isn't it - so memory is saved here constanly...

Thanks once again.

Message was edited by: java_cat33 - added Transparent Page Sharing

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kjb007
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Yes, as far as capacity planning goes, we are definitely on the same page, and allocating the additional memory should not be a problem. Transparent page sharing is on by default, and the more you use templates, the better your sharing will get.

I just wanted to clear up the slot calculation thing, as it can cause your cluster to go red, even though you know there are enough resources available to service the vm's you have running. Personally, I've set my slot size to 150 as it was in 3.0.2, and I do my capacity planning myself, and don't rely on slot calculation to tell me how much capacity I have available to me. I guess it's the control factor, but there's only so much I like giving up to automation, and still like to do some things on my own.

Anyway, you are good to go.

-KjB

vExpert/VCP/VCAP vmwise.com / @vmwise -KjB
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java_cat33
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Cheers for the clarification.

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