Host memory and Guest memory

[MariusRoma]

Consider an ESXi 5.x host with some guest VM, ,ember of a vSphere infrastructure.

Total host memory is about 85% of the physical RAM of the ESXi host, while the average guerst memory of guest VMs is around 10-15%.

So, the total guest memory is about 10-12% of the physical RAM.

Can I safely add guest VM and make the total host memory grow to 100% and more of the available physical RAM?

Given I don't have CPU problems and I don't want to see performance decrease, what percentage of either total host memory and total guest memory I must consider as the upper limit not to be exceeded?

I do realize that performance monitoring is a complex task and that each implementation is different from others, burt I am just looking for a general hint to avoid either wasting money to oversize RAM and face performance problems due to a downsized RAM...

Suggestions, pointers and hints welcome...

Regards

marius

[eeg3]

If your guest memory is that low, you should re-assess why the guests have so much more memory than they need. You will not want to put your host memory that high. Right-sizing the VMs will lower the host memory and allow you to add more VMs.

[rickardnobel]

Marius - Roma wrote:

Total host memory is about 85% of the physical RAM of the ESXi host, while the average guerst memory of guest VMs is around 10-15%.

So, the total guest memory is about 10-12% of the physical RAM.

Which specific counters do you observe for the guest memory?

There is, in my opinion, a risk if only looking at "active guest memory", which is a sample of how much memory has been very recently read or written from inside the VM. Very often this is a much smaller part of what pages the VM has actually loaded into RAM.

For example, say a VM with 4 GB RAM and running some kind of database. It has likely a lot of operating systems drivers, services and other code into RAM as well as the DB enginge and a lot of the database itself. This could consume all 4 GB. If looking at "active" memory from vSphere Client you might see only 500 MB of this as "active", meaning recently touched.

This is in my opinion not certain that this VM would work as well with a much lower memory configuration. It could very well do it, but it might also lead to a performance decrease.

[eeg3]

Great point by rickardnobel. There are many tools out there by vendors such as Veeam that will help right-size the environment based on greater statistical data if you need assistance in properly identifying actual usage, as well.