Re: [Qemu-devel] [PATCH 2/2] nbd-client: enable TCP keepalive

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

On Wed, Jun 05, 2019 at 09:39:10AM -0500, Eric Blake wrote:
> On 6/5/19 5:09 AM, Vladimir Sementsov-Ogievskiy wrote:
> > Enable keepalive option to track server availablity.
> 
> s/availablity/availability/
> 
> Do we want this unconditionally, or should it be an option (and hence
> exposed over QMP)?

I guess this is really a question about what our intended connection
reliability policy should be.

By enabling TCP keepalives we are explicitly making the connection
less reliable by forcing it to be terminated when keepalive
threshold triggers, instead of waiting longer for TCP to recover.

The rationale s that once a connection has been in a hung state for
so long that keepalive triggers, its (hopefully) not useful to the
mgmt app to carry on waiting anyway.

If the connection is terminated by keepalive & the mgmt app then
spawns a new client to carry on with the work, what are the risks
involved ? eg Could packets from the stuck, terminated, connection
suddenly arrive later and trigger I/O with outdated data payload ?

I guess this is no different a situation from an app explicitly
killing the QEMU NBD client process instead & spawning a new one.

I'm still feeling a little uneasy about enabling it unconditionally
though, since pretty much everything I know which supports keepalives
has a way to turn them on/off at least, even if you can't tune the
individual timer settings.

> > Requested-by: Denis V. Lunev <den@openvz.org>
> > Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> > ---
> >  block/nbd-client.c | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/block/nbd-client.c b/block/nbd-client.c
> > index 790ecc1ee1..b57cea8482 100644
> > --- a/block/nbd-client.c
> > +++ b/block/nbd-client.c
> > @@ -1137,6 +1137,7 @@ static int nbd_client_connect(BlockDriverState *bs,
> >  
> >      /* NBD handshake */
> >      logout("session init %s\n", export);
> > +    qio_channel_set_keepalive(QIO_CHANNEL(sioc), true, NULL);
> >      qio_channel_set_blocking(QIO_CHANNEL(sioc), true, NULL);
> >  
> >      client->info.request_sizes = true;
> > 
> 
> -- 
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3226
> Virtualization:  qemu.org | libvirt.org
> 




Regards,
Daniel
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