Is there really a limit to number of CPUs in ESXi?

[Juggler]

I've just installed the latest version of FreeNAS as a VM on my ESXi server. Reading through the documentation, I see mention that:

"Under “CPUs”, make sure that only 1 virtual processor is listed, otherwise you will be unable to start any FreeNAS® services."​

I only read this AFTER I had installed and run FreeNAS–I've configured mine with 8 virtual CPUs. Is there really a limit?

 

[jgreco]

mod note: moved to off-topic

Let me just start by asking why the hell you'd configure it with eight virtual CPU's. This is a frighteningly naive thing to do from a virtualization perspective; you do not just go throwing random numbers of CPU's at a VM because the virtualization platform will then starve your VM if that many cores are not currently available to handle your workload.

ESXi is twitchy about this sort of thing, so you're better off starting off with one core and then increasing to maybe two if you are seeing performance problems. We've got one instance of FreeNAS on a VM with four cores but the host machine has like two dozen so there's little chance of starvation.

 

[depasseg]

I'm not sure why or what services won't start in a multiple vCPU environment. Especially considering that running on a physical host pretty much guarantees running with multiple cores.

Great presentation on why over-provisioning CPU and RAM can hurt performance:
https://youtu.be/DMZD0kOCoYY?t=23m38s

 

[jgreco]

Especially considering that running on a physical host pretty much guarantees running with multiple cores.

I happen to think it's a reference to certain virtualization-related issues that have happened in the past, such as how MSI/MSI-X handling changes as you go from UP to MP.

Speaking as the resident virtualization "goo-rue", I can tell you that there've been a variety of issues, but I consider them to be more of a "try it and then figure a workaround if what you try doesn't work."

As for the cores thing: On a host platform that only has maybe 8 cores, asking for simultaneous access to even two cores can represent a significant availability impediment, and so unless you *NEED* two cores, you're better off asking for just one. The moment you add that second core, you need to make sure the ESXi host is not horribly busy or else performance will actually suffer. I haven't watched @depasseg 's linked video but I'm guessing he pointed you at a good discussion of this.

 

[nanodec]

more CPUS!

Must have more!!!!!

makes FreeNAS run in parallel, so your IOPS are more!

As an added bonus, if you are using enough CPUS then you don't need as much RAM...

Or add more RAM and run Windows in a jail...

BAM!

your entire network has been consolidated!

;)

 

[nanodec]

ok... i officially need to lay off the caffeine...

 

[nanodec]

hrm... although makes me wonder if I could run ESXi as a VM...

hehe... the matrix....

 

[gpsguy]

Sure you can, though I would only try it in a lab environment. Performance will probably tank.

I did it in the past - it might have been back in the day, when ESXi 5.0 was released.

Some VMware users have been known to test clustering, by virtualizing a couple of ESXi hosts on top of ESXi. Don't expect your guests to perform well.

hrm... although makes me wonder if I could run ESXi as a VM...
....

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