Re: [Qemu-devel] [PATCH 1/2] Add virtagent file system freeze/thaw

[Michael Roth]

On 02/01/2011 08:41 AM, Stefan Hajnoczi wrote:
> On Tue, Feb 1, 2011 at 2:36 PM, Jes Sorensen<address@hidden>  wrote:
> > On 02/01/11 15:34, Stefan Hajnoczi wrote:
> > > On Tue, Feb 1, 2011 at 2:26 PM, Jes Sorensen<address@hidden>  wrote:
> > > > I have to admit you lost me here, where do you get that 500ms time from?
> > > > Is that the XMLRPC polling time or? I just used the example code from
> > > > other agent calls.
> > >
> > > 500 ms is made up.  I was thinking, "what would a reasonable polling
> > > interval be?" and picked a sub-second number.
> > >
> > > Can you explain how the timeout in fsfreeze can happen?  It's probably
> > > because I don't know the virtagent details.
> >
> > Ah ok.
> >
> >  From what I understand, the XMLRPC code is setup to timeout if the guest
> > doesn't reply within a certain amount of time. In that case, the caller
> > needs to poll to wait for the guest to complete the freeze. This really
> > should only happen if you have a guest with a large number of very large
> > file systems. I don't know how likely it is to happen in real life.
>
> Perhaps Michael can confirm that the freeze function continues to
> execute after timeout but the client is able to send fsstatus()
> requests?

Ahh, yeah there's the confusion: we only execute one RPC at a time, so a 
polling function for a previous RPC won't work unless that RPC is being 
done concurrently, via fork()ing or something and communicating status 
via some method of IPC.

I touched on possible approaches to dealing with this in the response I 
just sent to this patch.

> Stefan