[ovirt-users] Need some help understanding memory allocation

[Martin Sivak]

Hi,

> So I am a bit confused as to the Ram being reported to me by the system
> (15.5GGB, but the system only has 12GB)

Let me start by asking you to check Cluster / Edit / Optimization tab
to see whether you have memory  overcommit enabled (it seems so).

You can also select the host from the webadmin and you will see all
the necessary information on the General subtab - most notably
Physical RAM metrics and Maximum free memory available for scheduling.
Those are a bit more accurate than the dashboard..

> Currently I am only running 3 VMs with memory allocations as follows:
> Or 6144MB used by VM

Add 256 MB reserved by default for the host operating system (in
/etc/vdsm/vdsm.conf) and 64 MB per VM for qemu overhead.

> If I SSH into my ovirt host, and run TOP and sort by memory, the top usage
> items are:
> 31896 qemu      21.5% 727:21.44 qemu-kvm
>  6008 qemu        14.6% 190:33.70 qemu-kvm
>  3019 ovirt            13.6%  35:47.64 java
>  5800 qemu         8.5% 784:26.88 qemu-kvm
>  2565 ovirt            3.9%  62:58.00 java

The engine takes both logical VM memory allocation (6GB) and the
physical memory into account to make sure the VMs will have enough
memory even in the future. So the displayed number usually does not
resemble what you see directly on the host, because the VMs are not
using their full share most of the time.

> This is a bare minimal install of CentOS7 running only Webmin and oVirt.

Do you have all your VMs on the same host as the webadmin management
console? That is usually a bad idea and we recommend using self hosted
engine for this kind of setup (the webadmin running in a VM).

> does a bare minimal install of CentOS really need 4-5GB of Ram to run?

Nope. It should be couple of hundred megabytes max for the host
services. Check what `free` tells you too, I am pretty sure you will
have a lot of cached pages there.


This is all I can tell right now, the numbers from the Host / General
subtab and the info about overcommit would shed more light on the
topic.

Best regards

Martin Sivak



On Thu, Aug 10, 2017 at 11:23 PM, Wesley Stewart <wstewart3 at gmail.com> wrote:
> I am a little confused on why my CentOS box is using so much RAM.  I
> currently have a small test setup which only has 12GB of ram total.
>
> When I log into the administrative interface it tells me:
>
> Memory
>
> 4.3
>
> Available
> of 15.5 GiB
> Over commit: 39% (allocated 45%)
>
> 11.1 GB Used
>
> Currently I am only running 3 VMs with memory allocations as follows:
>
> Defined Memory:
> 1024 MB
> Physical Memory Guaranteed:
> 1024 MB
>
> Defined Memory:
> 3072 MB
> Physical Memory Guaranteed:
> 3072 MB
>
> Defined Memory:
> 2048 MB
> Physical Memory Guaranteed:
> 2048 MB
> Or 6144MB used by VM
>
> If I SSH into my ovirt host, and run TOP and sort by memory, the top usage
> items are:
> 31896 qemu      21.5% 727:21.44 qemu-kvm
>  6008 qemu        14.6% 190:33.70 qemu-kvm
>  3019 ovirt            13.6%  35:47.64 java
>  5800 qemu         8.5% 784:26.88 qemu-kvm
>  2565 ovirt            3.9%  62:58.00 java
>
> Everything else is pretty small compared, but for the OVIRT/QEMU we roughly
> get
> Total: 62% = 7.45GB
>
> So I am a bit confused as to the Ram being reported to me by the system
> (15.5GGB, but the system only has 12GB), and assuming OVIRT is using around
> 7-8GB, does a bare minimal install of CentOS really need 4-5GB of Ram to
> run?
>
> This is a bare minimal install of CentOS7 running only Webmin and oVirt.  I
> think there are several things I don't understand here, and I am quite new
> to oVirt and would like to learn, so please excuse my inexperience if this
> is a silly question!
>
> Thanks either way, I am really happy I ditched my ESXi box :D
>
>
>
>
> _______________________________________________
> Users mailing list
> Users at ovirt.org
> http://lists.ovirt.org/mailman/listinfo/users
>