Re: [Xen-devel] [PATCH 4 of 4 V5] tools/xl: Remus - Network buffering cmdline switch

[Brendan Cully <brendan@xxxxxxxxx>]

On Thursday, 12 December 2013 at 10:37, Shriram Rajagopalan wrote:
> On Thu, Dec 12, 2013 at 7:12 AM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>wrote:
> 
> > Shriram Rajagopalan writes ("Re: [PATCH 4 of 4 V5] tools/xl: Remus -
> > Network buffering cmdline switch"):
> > > On Mon, Nov 25, 2013 at 7:37 AM, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
> > wrote:
> > >     Shriram Rajagopalan writes ("[PATCH 4 of 4 V5] tools/xl: Remus -
> > Network
> > >     buffering cmdline switch"):
> > >     > tools/xl: Remus - Network buffering cmdline switch
> > ...
> > >     Shouldn't enabling network buffering be the default ?
> > >
> >
> > > Given that network buffering support is conditionally compiled into
> > > libxl, it makes sense to let the user explicitly enable network
> > > buffering.  If network buffering was enabled by default, but libxl
> > > was missing the feature, then running xl remus (without any option)
> > > would immediately fail saying network buffering support is
> > > missing. Might as well make people enable it explicitly and then
> > > discover the lack of support for it inside libxl.
> >
> > Is Remus actually useable without network buffering ?  I don't think
> > we would want to change the default later, and clearly the default
> > should be to behave correctly.
> >
> 
> It is usable without network buffering. Memory state is preserved on
> failover.
> On failover, without network buffering, exising network
> connectivity (tcp connections) "may" be lost, under some circumstances.

uh, wouldn't it be possible to acknowledge client requests whose
backing state hasn't been replicated? So that on failover the backup
might have no memory of a promised it had made? it's more than just
tcp resets, isn't it?

> You are right about the fact that for Remus to behave 'correctly', one
> needs both network and disk buffering -- the latter being absent is the
> reason
> Remus in libxl is still experimental.
> 
> So, might as well enable network buffering by default, punt if the support
> is
> missing and suggest that the user invoke Remus without network buffering
> (while providing sufficient warnings that the failover may be incomplete)
> 
> 
> > >     Is it really useful to have a command-line option to change the
> > script ?
> > >
> > > That was added on Ian C's suggestion. It might be useful if people have
> > > openvswitch based setups and want to tweak the network buffering setup.
> >
> > I think this should be a per-vif configuration option, with a default
> > taken from the domain and/or global config.  Not a command-line
> > option.
> >
> >
> This is getting needlessly complicated. These are "nice to have" features,
> which I am least concerned about at this moment.
> I am more than happy to fall back to simply using the default script
> specified in xl.conf,
> for the initial version.  If users want to provide a per-vif configuration
> script, the support
> can be integrated later.
> 
> 
> > Thanks,
> > Ian.
> >
> >

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