Wrong memory usage ?

[Alessandro 123]

Hi guys,
I have a PVE node with these VM running on it:


So, there are 2+2+1+16GB allocated. Total 21GB

But why here I can see 51.96GB used ?

 

[LnxBil]

But why here I can see 51.96GB used ?

Caching (including ZFS ARC) and VM overhead.

 

[oguz]

hi,

what's the output from free -h?

i see you're also using ZFS which adds to the cached memory for ARC [0]

[0]: https://pve.proxmox.com/wiki/ZFS_on_Linux#sysadmin_zfs_limit_memory_usage

 

[Alessandro 123]

Should I reduce the ZFS ARC cache manually or it does it on its own ?
In other words, can I add 2 VM with 16GB each ?

 

[LnxBil]

Should I reduce the ZFS ARC cache manually or it does it on its own ?

There are reports that the host might run into OOM problems, so you should reduce it to a size that is comfortably for you if want to go down that road.

In other words, can I add 2 VM with 16GB each ?

Yes, but you will run into serious memory problems and things will start to slow down. Each VM needs more memory that you allocate to it due to the memory required for the processes around your VMs. You're already using KSM (kernel samepage merging) that - as the name suggests - deduplicates memory. If you add two VMs with the same OS (same patch level) you may see an increase in merged pages which yields in more VMs running with the same memory.

Please also check if you use caching for your harddisks in each VM -> Hardware -> Harddisk -> edit and set to "none" if not already set to this.

 

[Alessandro 123]

Yes, but you will run into serious memory problems and things will start to slow down. Each VM needs more memory that you allocate to it due to the memory required for the processes around your VMs. You're already using KSM (kernel samepage merging) that - as the name suggests - deduplicates memory. If you add two VMs with the same OS (same patch level) you may see an increase in merged pages which yields in more VMs running with the same memory.

well, adding 2 VMs more, means a total RAM usage of 52GB on a 64GB host. There are 12GB free for the host.

 

[LnxBil]

well, adding 2 VMs more, means a total RAM usage of 52GB on a 64GB host. There are 12GB free for the host.

on the highest abstraction level ... maybe ... but it is much more complicated than that. As I already tried to explain, a VM needs more memory than just the memory you allocate for the VM. This depends on a lot of things and cannot be easily determined. e.g. you're already using much more than the sum of the VMs and this "collateral ram usage" will only increase with more VMs until the point that you will constantly read from disk or swap and then eventually OOM. And I don't even talk about slab allocation ... memory management can be VERY complicated.