On Wed, Feb 11, 2015 at 12:17 PM, Chris Adams <cma@cmadams.net> wrote:

Once upon a time, Chris Adams <cma@cmadams.net> said:
> Once upon a time, Dan Yasny <dyasny@gmail.com> said:
> > shouldn't be hard to do. Can you provide the details of what you need to
> > happen to the VM exactly?
> > - domxml changes
> > - other host level changes
> > - whether the VM should be able to live migrate
>
> It looks like libvirt supports setting up DSCP bits with nwfilter, per:
>
> https://libvirt.org/formatnwfilter.html

Oh, on reading this, nwfilter can only match, not set, so that won't
help. It doesn't look like libvirt has a way to set something like
that.

Do VDSM hooks only act on the XML, or is there a way to configure things
outside of libvirt?

The hooks are point is the VMs lifetime where you can insert any code you like.

There is a way to edit the domxml at certain points, because it hasn't been passed

to libvirt for execution yet. At other points changing the domxml will do nothing

useful because the VM is already running, but you can do other things, execute

other commands. A classic example is an oVirt independent Vm event monitoring

engine, to which you can issue a hook based message at certain lifecycle points,

e.g. "VM $VM is starting"; "VM $VM is migrating" etc. No idea why you'd need

that engine (it's quite redundant because the ovirt-engine already does all this)

but it will definitely work.

In short, explore the hooks, note the livecycle points (they are simply aptly named

directories on the hosts) and decide what and where you need to execute.

If you are modifying a VM, it probably makes sense to do it in before_vm_start, but

if you're also live migrating that VM, you need to add the same changes in

before_migrate_vm_*, and if the hook script makes changes on the host, you

probably want to clean up after the VM is gone, so after_vm_stop and

after_vm_migrate_source are the places to start looking. The mechanism is very

flexible really, you can do anything you want if the host OS is capable of it

--
Chris Adams <cma@cmadams.net>
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