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Date:	Wed, 17 Nov 2010 12:14:47 +0100
From:	Tejun Heo <tj@...nel.org>
To:	Dan Smith <danms@...ibm.com>
CC:	Gene Cooperman <gene@....neu.edu>,
	Oren Laadan <orenl@...columbia.edu>,
	Kapil Arya <kapil@....neu.edu>,
	ksummit-2010-discuss@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org, hch@....de
Subject: Re: [Ksummit-2010-discuss] checkpoint-restart: naked patch

Hello,

On 11/08/2010 08:05 PM, Dan Smith wrote:
> GC> As before, Oren, let's have that phone discussion so that we can
> GC> preprocess a lot of this, instead of acting like the the three
> GC> blind men and the elephant.  I will _tell you_ the strengths and
> GC> weaknesses of DMTCP on the phone, instead of you having to guess
> GC> at them here on LKML.  And of course, I hope you will be similarly
> GC> frank about Linux C/R on the phone.
> 
> I want to be in on that discussion too, as do a lot of other people
> here.  However, I doubt we'll all be able to find a common spot on our
> collective schedules, nor would that conversation be archived for
> posterity.  I think sticking to LKML is the right (and time-tested)
> approach.

Amen.

> OL> Linux-cr can do live migration - e.g. VDI, move the desktop - in
> OL> which case skype's sockets' network stacks are reconstructed,
> OL> transparently to both skype (local apps) and the peer (remote
> OL> apps).  Then, at the destination host and skype continues to work.
> 
> GC> That's a really cool thing to do, and it's definitely not part of
> GC> what DMTCP does.  It might be possible to do userland live
> GC> migration, but it's definitely not part of our current scope.
> 
> How would you go about doing that in userland?  With the current
> linux-cr implementation, I can move something like sshd or sendmail
> from one machine to another without a remote (connected) client
> noticing anything more than a bit of delay during the move.
> 
> I think that saving and restoring the state of a TCP connection from
> userland is probably a good example of a case where it makes sense to
> have it as part of a C/R function, but not necessarily exposed in /sys
> or /proc somewhere.  Unless it can be argued that doing so is not
> useful, I think that's a good talking point for discussing the kernel
> vs. user approach, no?

Meh, just implementing a conntrack module should be good enough for
most use cases.  If it ever becomes a general enough problem (which I
extremely strongly doubt), we can think about allowing processes in a
netns to change sequence number but that would be a single setsockopt
option instead of the horror show of dumping in-kernel data structures
in binary blob.

Thanks.

-- 
tejun
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