Oversized VM

[DanieleFiore201]

Dear guys,

i wanna post another example of a monster VM with full reservation and his behavior in terms mermory overhead.

I still have my monster VM with 512GB of RAM and 32vCPU.

Application vendor require memory reservation therefore we assigned full memory reserved.

Of course it cannot start but that condition remains until i reduce the reservation of 100GB at least.

So in the end to start the VM i need to reserve only  420 GB instead of 512.

When i check the overhead it is 117GB......

Host is 512GB physical ram + 20 vCPU.

I read many articles , including some about oversized VMs that can bring to decrease performance, but i cannot find something that justifies with my customer why we "loose" 100GB of RAM for overhead.

Any suggestion ?

many thanks ,

Daniele

[spravtek]

Why justify?

Memory Overhead is the memory the VMkernel thinks it will need to run the VM workload.

So ESXi thinks it will need 100Gb (or more), this normally includes things like page tables, frame buffers...

In other words, there's no way around it you could say ...

Is this 117Gb the amount that is actually in use (you see it on the Resource Allocation tab of the VM - Overhead consumption).

[DanieleFiore201]

Hi thanks for your reply.

I say justify as the customer is saying it cannot accept to have such high overhead....he has a 512 GB host and he wants to use it all (or at least not losing the 25%).

So are you saying for you this is correct ?

The thing is if i remove the servervation such overhead goes away and i'm able to start the VM.

[spravtek · Accepted solution]

This is correct yes ... The more memory and vCpu's your VM has, the higher the memory overhead will be ...

See page 30 of this document for more info: http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-resourc...

Or these blog posts:

http://frankdenneman.nl/memory/virtual-machine-memory-overhead/

http://simongreaves.co.uk/blog/virtual-machine-memory-overhead