Re: [Qemu-devel] [PATCH 0/3] Migration cancel with dead network

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jan 08, 2015 at 11:29:59AM +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrange (berrange@redhat.com) wrote:
> > On Thu, Jan 08, 2015 at 11:11:29AM +0000, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > If the remote host, or networking dies during a migration, the socket can be
> > > waiting for a long timeout, and migration_cancel can't complete the cancel
> > > for a long time (and you can't start a new one to somewhere else).
> > > (Where 'long' is the TCP timeout, that's ~15 mins)
> > > 
> > > This patch set uses the shutdown(2) syscall to unblock any write/sends that
> > > are in progress to let the migrate_cancel happen quickly.
> > > 
> > >   1/3: socket shutdown  - An updated patch from my postcopy world to
> > >                           add a shut_down method on QEMUFile - only
> > >                           for 'socket' (where the syscall is supported).
> > > 
> > >   2/3: Handle bi-directional communication for fd migration
> > >                         - A patch from Cristian Klein to use the socket
> > >                           QEMUFile for FDs that are passed in, if the FDs
> > >                           are sockets; this is needed so that libvirt
> > >                           migrations can take advantage of the other patches.
> > >                           Again this patch (and its naming) come from the
> > >                           postcopy world.
> > > 
> > >   3/3: migration_cancel: shutdown migration socket
> > >                         - A new patch that uses the shutdown in migrate_fd_cancel
> > > 
> > > 
> > > Note this does not fix the timeout if you try to migrate to an already dead host;
> > > the connect timeout is typically a much shorter 2 minutes anyway.
> > 
> > In any libvirt managed setup, you'd need to address the connect timeout
> > issue in libvirt instead, since libvirt always uses fd based migration.
> > ie libvirt estabishes the connection & then passes the TCP Socket to
> > QEMU.
> > 
> > It should be possible for libvirt to use a non-blocking connect()
> > call and catch use of its virDomainMigrateCancel APi to stop the
> > connection attempt.
> 
> Yes, although as I say I've not fixed the (must shorter) connect side timeout
> on the QEMU side either.
> 
> How does libvirt behave if you cancel a tunnelled migration with the network
> dieing in the middle?

I think its just reliant on the TCP timeout - we don't have any of the
clever use of 'shutdown' that you're adding here. I guess it could make
sense for libvirt to use shutdown.

Regards,
Daniel
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