linux-kernel - Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2: hang in atomic copy)

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Date:	Wed, 25 Apr 2007 15:55:58 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nigel Cunningham <nigel@...el.suspend2.net>
cc:	Pavel Machek <pavel@....cz>, Kenneth Crudup <kenny@...ix.com>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Con Kolivas <kernel@...ivas.org>,
	suspend2-devel@...ts.suspend2.net, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2:
 hang in atomic copy)

On Thu, 26 Apr 2007, Nigel Cunningham wrote:
> > 
> > And name *one* thing that have in common.
> 
> Set/reset the scsi transaction id thingy? Hibernation didn't work with
> SCSI for a long time precisely because that support was missing.

And by "hibernation", you mean what? You mean "snapshot + shutdown", 
right?

Think about it for five seconds, and then ask yourself: at which point in 
the "snapshot + shutdown" sequence would you actually tell a disk to shut 
down?

If you said "snapshot", then you'd be *wrong*. 

That's my _point_. The snapshot() function should not (and MUST NOT) tell 
disks to shut down, because unlike suspend(), we're still going to _use_ 
those disks afterwards (why? To write out the snapshot image!).

In other words, the act of creating a snapshot has *nothing* to do with 
suspend.

Now, after you've created (and written out) the snapshot, what do you 
actually end up doing?

That's right - you end up _shutting down_ the machine, and yes, as part 
of the _shutdown_ sequence you may actually end up doing a lot of the 
things that a suspend would do. But that's long *after* you've actually 
done the "snapshot" part, and has absolutely nothing to do with it.

That's where I started: whole "suspend to disk" thing actually has _more_ 
to do with "shutdown" than with "suspend". 

			Linus
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