[Libvir] Adding suppport for daemon restarts with stateful drivers

[Daniel Veillard]

On Fri, Mar 21, 2008 at 05:35:12PM +0000, Daniel P. Berrange wrote:
> The libvirt daemon has the ability to reload itself by sending it SIGHUP. 
> For the QEMU & network drivers this makes it reload the config files for
> VMs and re-init the iptables rules. It would be desirable though to allow
> the daemon to perform a full restart. Principally this is for RPM upgrades
> where you want toensure the daemon is running the new code.
> 
> The tricky thing is figuring out how to handle driver state. Looking at the
> QEMU, network, storage and LXC drivers, there is not actually all that  much
> state to deal with. It basically comes down to:
> 
>  - PID of child processes (eg QEMU, dnsmasq, container)
>  - FDs for STDIN/OUT/ERR of the child processes
>  - A possible logfile FD
>  - Flag to indicate whether some objects are active or not
> 
> That is more or less it. Anything else is kept in the config files and can
> be reloaded at will.

  From a libvirt client connected to the driver POV we would still either
see a disconnection or a potential loss of state depending how they are
connected, right ? if we are sure we can transparently restart fine, but
I'm not sure it's always the case for say an ssh connection without an agent,
still being able to re-exec on the new code is important, I would still try to
avoid it if we can detect the code itself didn't change (for example if the
timestamp on the /usr/sbin/libvirtd didn't change it's likely to be a simple
reload -HUP command) 

> So I was thinking about whether we could provide a simple protocol to allow
> each stateful driver to save its state into some location, the daemon could
> just 'exec()' itself again, and upon startup the drivers reload their active
> state. Since the daemon just exec()'s itself it would still own the child
> processes & still have all the neccessary FD's open. 

 yes but how much state is kept in buffers and code of the protocols ?

Daniel

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