About host memory - does maxing = crashing?

[rightfoot]

Say a host has 64GB of memory and vms are using up to 60GB for example.

Does it mean I can still use the rest of the 4GB?

What I mean is, does the host take what ever it needs and what ever is left is what is given to the vms to share?

If I max out the memory beyond sharing it between vms, can the host end up crashing?

[TheBobkin · Accepted solution]

Hello,

Short answer - No, this is extremely unlikely (VMs would likely hang or crash a long time before this could happen).

ESXi hosts use a multitude of methods to allow over-provisioning of memory to VMs above the physical memory including:

- Memory sharing (as similar workload/Guest OS VMs will have an amount of identical data).

- Ballooning (reclaiming least used memory from VMs using VMware Tools provided this is installed on the VM).

- Memory compression (as it is faster to compress and read than read from disk if it were have to resort to swapping).

- Swapping (swapping out memory to disk into the main .vswp file of the VM, this is a last resort and impcacts performance).

p27-44 of this explains all of these concepts better:

https://docs.vmware.com/en/VMware-vSphere/6.0/vsphere-esxi-vcenter-server-601-resource-management-gu...

More info:

https://docs.vmware.com/en/VMware-vSphere/5.5/com.vmware.vsphere.resmgmt.doc/GUID-9D2D0E45-D741-476F...

You can likely use more than 60GB here but VM performance may impacted as it has to use the more aggressive methods.

(check if/how much you are have as memory reservation for VMs as this can end up reserved and will not be reclaimed using some of the above methods, check that VMs have VMware Tools installed).

Bob

[rightfoot]

The memory between vms is definitely well shared meaning, we never see any spikes to the point where many vms use up all of the memory. Some have more than others meaning that if they all used it up, I'd have a problem but that never seems to happen since I try to balance what the vms are and do.

So basically, it sounds like we're fine and I now have a better understanding of memory use.

Thanks

[TheBobkin]

Hello rightfoot,

Well in a situation that VMs 'all used it up' it will start using the methods briefly described in my last comment with preference of the least impactful.

So to that end - ensure VMs at least have a recent version of VMware Tools installed (so it can use ballooning which is vastly preferable over swapping to disk) and only have memory reservation of reasonable amounts on critical VMs (using Resource Pools and shares is preferable to reservations, though this require Enterprise or Enterprise+ licensing iirc).

Bob