FN 11 VM resource allocation question.

pyrojoe34 · July 16, 2017

Quick question regarding resource allocation of a VM running within FreeNAS 11. Sorry if this is a dumb question, I'm new to using VMs.

So the main OS on the system is FN11 and I installed a Ubuntu VM that gets 4 (out of 8) CPU cores and 6GB RAM (out of 16). Does this mean that FN11 now only has access to 4 cores and 10GB or are the resources dynamically allocated as needed and the settings only represent what the VM is allowed to use/see? For example: If the VM is given 6GB but is only using 1GB at the time, does this mean that FN11 can still use 15GB or is it still limited to the 10GB? Could I theoretically give a VM all 8 cores and still have the rest of the system function normally? Side question: can VMs overlap resources? Can I have 3 VMs running each with 4 cores even though there are only 8 cores total?

Thanks for your help.

dalekphalm · July 16, 2017

You can assign more resources to the VM's and the host will still "see" all the ram and cores. If a VM was allocated 4 cores but is only currently utilizing 1 core, the host (or another VM) can take advantage of those unused cores.

However, you want to be careful. You want to look at peak performance and resource utilization. If each VM uses all their cores and the server doesn't have enough physical resources to feed them all, you can run into huge performance issues.

However, if not all VM's will be running at once, or you can guarantee that they won't use too much and overlap, then what you propose is fine.

Personally, I avoid assigning too many cores. I have 16 thread in my server and I won't assign more than 16 "cores" out in total.

Mikensan · July 17, 2017

Seldom will you need more than 2 cpu/cores in a vm, so unless you a hit a bottleneck I'd start with 2.

FreeNAS 11 uses behyve for vms, so how it shares resources is dependent on that. Normally if cpu cycles are free then the host has access to them, but the vm will get priority. Since FreeNAS will use any available RAM for cache, behyve may reserve RAM for the VMs, not really sure tbh.

One issue with over-allocating CPUs to a system is that at least in Windows, the O/S will try to distribute load across all available cores which will tie them up.