Re: [PATCH 2/2] util/qemu-sockets: make keep-alive enabled by default

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

On Thu, Jul 09, 2020 at 11:49:17AM +0300, Vladimir Sementsov-Ogievskiy wrote:
> 09.07.2020 11:29, Daniel P. Berrangé wrote:
> > On Wed, Jul 08, 2020 at 10:15:39PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> > > Keep-alive won't hurt, let's try to enable it even if not requested by
> > > user.
> > 
> > Keep-alive intentionally breaks TCP connections earlier than normal
> > in face of transient networking problems.
> > 
> > The question is more about which type of pain is more desirable. A
> > stall in the network connection (for a potentially very long time),
> > or an intentionally broken socket.
> > 
> > I'm not at all convinced it is a good idea to intentionally break
> > /all/ QEMU sockets in the face of transient problems, even if the
> > problems last for 2 hours or more.
> > 
> > I could see keep-alives being ok on some QEMU socket. For example
> > VNC/SPICE clients, as there is no downside to proactively culling
> > them as they can trivially reconnect. Migration too is quite
> > reasonable to use keep alives, as you generally want migration to
> > run to completion in a short amount of time, and aborting migration
> > needs to be safe no matter what.
> > 
> > Breaking chardevs or block devices or network devices that use
> > QEMU sockets though will be disruptive. The only solution once
> > those backends have a dead socket is going to be to kill QEMU
> > and cold-boot the VM again.
> > 
> 
> Reasonable, thanks for explanation.
> 
> We are mostly interested in keep-alive for migration and NBD connections.
> (NBD driver has ability to reconnect). What do you think about setting
> keep-alive (with some KEEPIDLE smaller than 2 hours) by default for
> migration and NBD (at least when NBD reconnect is enabled), would it be
> valid?

I think it should be reasonable to set by default for those particular
scenarios, as both are expecting failures and ready to take action when
they occur.

Regards,
Daniel
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