linux-kernel - Re: C/R without "leaks"

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Date:	Fri, 17 Apr 2009 11:15:46 +0200
From:	Greg Kurz <gkurz@...ibm.com>
To:	Oren Laadan <orenl@...columbia.edu>
Cc:	Chris Friesen <cfriesen@...tel.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Linux-Kernel <linux-kernel@...r.kernel.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	containers@...ts.osdl.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: C/R without "leaks"

On Thu, 2009-04-16 at 14:39 -0400, Oren Laadan wrote:
> Any connection in that case is, of course, lost, and it's up to the
> application to do something about it. If the application relies on
> the state of the connection, it will have to give up (e.g. sshd, and
> ssh, die).
> 

And that's a good thing since that's exactly what users expect from
sshd : to give up the connection when something goes wrong. I wouldn't
trust a sshd with the ability to initiate connections on its own...

And anyway, I still don't see the scenario where C/R a sshd is useful...
Please someone (Alexey ?), provide a detailed use case where people
would want to checkpoint or migrate live TCP connections... Discussion
on containers@ is very interesting but really lacks of
what-is-the-bigger-picture arguments... These huge patchsets are very
tricky and intrusive... who wants them mainline ? what's the use of
C/R ?

> However, there are many application that can withstand connection
> lost without crashing. They simply retry (web browser, irc client,
> db clients). With time, there may be more applications that are
> 'c/r-aware'.
> 

HPC jobs are definitely good candidates.

> Moreover, in some cases you could, on restart, use a wrapper to
> create a new connection to somewhere (*), then ask restart(2) to
> use that socket instead of the original, such that from the user
> point of view things continue to work well, transparently.
> 

Yes.

> (*) that somewhere, could be the original peer, or another server,
> if it has a way to somehow continue a cut connection, or a special
> wrapper server that you right for that purpose.
> 
> Oren.
> 
-- 
Gregory Kurz                                     gkurz@...ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)534 638 479                           Fax +33 (0)561 400 420

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.

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