[ovirt-users] VDSM hook for setting DSCP bits?

[Dan Yasny]

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

> Once upon a time, Chris Adams <cma at cmadams.net> said:
> > Once upon a time, Dan Yasny <dyasny at 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 at cmadams.net>
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