[GTALUG] Setting up a VM host

[Lennart Sorensen]

On Sat, Aug 27, 2016 at 11:53:39AM -0400, Anthony de Boer via talk wrote:
> The other thing it doesn't do is give you a bunch of virtual machines
> with collectively more memory than the server has installed.  I've had
> to deal with a box where people had given away 110% of the RAM chips
> to their various QEMU instances and the server was crawling around on
> the floor in a very slow and hesitant way.

qemu/kvm supports balloon drivers in the guest.  Also the host could
run KMS.  So you have multiple options for allowing you to overprovision
the system and still do fine.

> Someone mentioned options: yes, QEMU has a lot of them, and having a
> script that sets the standard ones and lets you specify just what's
> different between each one and the next can make starting them a lot
> easier.  QEMU's native commandline can easily start onto a third line on
> an 80-column screen.
> 
> Also, do please use kernel bridging and not the userspace virtual
> switch.

Yes a kernel bridge with tap interfaces in qemu works pretty well.
There might even be better options than the tap interface these days.

> If you're running a bunch of heterogenous operating systems, though,
> QEMU gets you there.  I've even seen people use it for Windows.
> 
> If everything is this-decade Linux then LXC may be an option.  It
> isolates each VM's processes and root filesystem and network interfaces
> while still sharing memory, CPUs, and (optionally, if VMs are in the same
> filesystem) disk space.  I've run three generations of Debian all under
> the kernel that came with the latest.  If you're also wanting to run eg
> Fedora then there's a good chance of it just working, or if not you may
> have to compare kernel configs and build a host kernel that makes
> everyone happy.
> 
> LXC lets you run collectively a far bigger party; users have access to
> all the CPUs and RAM when they have a large compute job, not just the
> tiny ration the QEMU config sets aside for them.  So far I haven't had
> to configure resource limits on LXC; YMMV.

You can restrict resources with lxc as far as I recall.  But you might
not have to.  I guess it depends if you trust the stuff inside the
container to be well behaved or not.

-- 
Len Sorensen