Memory overhead on monsters VMs

[DanieleFiore201]

Hi guys,

i'm having a discussion with some application guys here who wants to implement a 512GB vRAM VM on a single 512RAM host (vSphere5).
They are not considering at all memory overhead.

Consider the VM is this size :

32vCPU and , at the moment with 412GB vRAM.

I do see an overhead of 3GB and that makes sense to me cause i remember the more vCPU and vRAM you have the more overhead is.

I just want to have some opinions how much vRAM we should assign to that monster VM.

I checked the following links that make absolutely sense to me :

http://pubs.vmware.com/vsphere-4-esx-vcenter/index.jsp?topic=/com.vmware.vsphere.resourcemanagement....

Any opinions ?

Many thanks in advance.

Daniele

[spravtek · Accepted solution]

Besides the memory overhead for the VM's, there's also the memory overhead for the VMkernel and host-agents ...

Also important to remember:

• You should allocate enough memory to hold the working set of applications you will run in the virtual

machine, thus minimizing thrashing.

• You should also avoid over-allocating memory. Allocating more memory than needed unnecessarily

increases the virtual machine memory overhead, thus consuming memory that could be used to support
more virtual machines.

Now maybe you're not going to run any more VM's on this one host, but still, the more you over-allocate, the more overhead.

Excerpt from: http://www.vmware.com/pdf/Perf_Best_Practices_vSphere5.0.pdf