linux-kernel - Re: Back to the future.

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Message-ID: <Pine.LNX.4.64.0704271303260.925@sbz-30.cs.Helsinki.FI>
Date:	Fri, 27 Apr 2007 13:12:08 +0300 (EEST)
From:	Pekka J Enberg <penberg@...helsinki.fi>
To:	Oliver Neukum <oliver@...kum.org>
cc:	Nigel Cunningham <nigel@...el.suspend2.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Back to the future.

Am Freitag, 27. April 2007 08:18 schrieb Pekka J Enberg:
> > No. The snapshot is just that. A snapshot in time. From kernel point of 
> > view, it doesn't matter one bit what when you did it or if the state has 
> > changed before you resume. It's up to userspace to make sure the user 
> > doesn't do real work while the snapshot is being written to disk and 
> > machine is shut down.

On Fri, 27 Apr 2007, Oliver Neukum wrote:
> And where is the benefit in that? How is such user space freezing logic
> simpler than having the kernel do the write?
>
> What can you do in user space if all filesystems are r/o that is worth the
> hassle?

I am talking about snapshot_system() here. It's not given that the 
filesystems need to be read-only (you can snapshot them too). The benefit 
here is that you can do whatever you want with the snapshot (encrypt, 
compress, send over the network)  and have a clean well-defined interface 
in the kernel. In addition, aborting the snapshot is simpler, simply 
munmap() the snapshot.

The problem with writing in the kernel is obvious: we need to add new code 
to the kernel for compression, encryption, and userspace interaction 
(graphical progress bar) that are important for user experience.

				Pekka
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