Re: Back to the future.

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To: Nigel Cunningham <nigel@...>
Cc: Pekka Enberg <penberg@...>, LKML <linux-kernel@...>
Date: Thursday, April 26, 2007 - 12:56 pm

On Thu, 26 Apr 2007, Nigel Cunningham wrote:

I'd actually like to discuss this a bit..

I'm obviously not a huge fan of the whole user/kernel level split and 
interfaces, but I actually do think that there is *one* split that makes 
sense:

 - generate the (whole) snapshot image entirely inside the kernel

 - do nothing else (ie no IO at all), and just export it as a single image 
   to user space (literally just mapping the pages into user space). 
   *one* interface. None of the "pretty UI update" crap. Just a single 
   system call:

	void *snapshot_system(u32 *size);

   which will map in the snapshot, return the mapped address and the size 
   (and if you want to support snapshots > 4GB, be my guest, but I suspect 
   you're actually *better* off just admitting that if you cannot shrink 
   the snapshot to less than 32 bits, it's not worth doing)

User space gets a fully running system, with that one process having that 
one image mapped into its address space. It can then compress/write/do 
whatever to that snapshot.

You need one other system call, of course, which is

	int resume_snapshot(void *snapshot, u32 size);

and for testing, you should be able to basically do

	u32 size;
	void *buffer = snapshot_system(&size);
	if (buffer != MAP_FAILED)
		resume_snapshot(buffer, size);

and it should obviously work.

And btw, the device model changes are a big part of this. Because I don't 
think it's even remotely debuggable with the full suspend/resume of the 
devices being part of generating the image! That freeze/snapshot/unfreeze 
sequence is likely a lot more debuggable, if only because freeze/unfreeze 
is actually a no-op for most devices, and snapshotting is trivial too.

Once you have that snapshot image in user space you can do anything you 
want. And again: you'd hav a fully working system: not any degradation 
*at*all*. If you're in X, then X will continue running etc even after the 
snapshotting, although obviously the snapshotting will have tried to page 
a lot of stuff out in order to make the snapshot smaller, so you'll likely 
be crawling.


I'd really suggest _just_ the "full image". Nothing else is probably ever 
worth supporting. Your "snapshot to disk" wouldn't be _quite_ as simple as 
"echo disk > /sys/power/state", but it should not necessarily be much 
worse than

	snapshot_kernel | gzip -9 > /dev/snapshot

either (and resuming from the snapshot would just be the reverse)!

And if you want to send the snapshot over a TCP connection to another 
host, be my guest. With pretty images while it's transferring. Whatever.

			Linus
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Messages in current thread:
Back to the future., Nigel Cunningham, (Thu Apr 26, 2:04 am)
Re: Back to the future., Jan Engelhardt, (Thu Apr 26, 4:38 am)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 5:33 am)
Re: Back to the future., Pekka Enberg, (Thu Apr 26, 3:28 am)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 3:42 am)
Re: Back to the future., Linus Torvalds, (Thu Apr 26, 12:56 pm)
Re: Back to the future., Chase Venters, (Thu Apr 26, 2:22 pm)
Re: Back to the future., David Lang, (Thu Apr 26, 2:50 pm)
Re: Back to the future., Linus Torvalds, (Thu Apr 26, 1:07 pm)
Re: Back to the future., Pavel Machek, (Fri Apr 27, 8:49 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 5:26 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 6:12 pm)
Re: Back to the future., Pavel Machek, (Thu Apr 26, 6:42 pm)
Re: Back to the future., David Lang, (Thu Apr 26, 6:24 pm)
Re: Back to the future., Pavel Machek, (Thu Apr 26, 7:12 pm)
Re: Back to the future., David Lang, (Thu Apr 26, 6:49 pm)
Re: Back to the future., Olivier Galibert, (Thu Apr 26, 8:23 pm)
Re: Back to the future., Pavel Machek, (Thu Apr 26, 7:27 pm)
Re: Back to the future., David Lang, (Thu Apr 26, 6:56 pm)
Re: Back to the future., Pavel Machek, (Thu Apr 26, 6:40 pm)
Re: Back to the future., Pekka Enberg, (Fri Apr 27, 1:41 am)
Re: Back to the future., Pavel Machek, (Fri Apr 27, 10:55 am)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 5:39 pm)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 3:56 pm)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 12:52 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 4:44 pm)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 2:08 am)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 2:18 am)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 2:29 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 5:24 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 5:44 pm)
Re: Back to the future., Jeremy Fitzhardinge, (Fri Apr 27, 8:18 pm)
Re: Back to the future., Matthew Garrett, (Fri Apr 27, 9:00 pm)
Re: Back to the future., Jeremy Fitzhardinge, (Fri Apr 27, 9:05 pm)
Re: Back to the future., Pavel Machek, (Thu May 3, 11:14 am)
Re: Back to the future., Eric W. Biederman, (Fri Jun 1, 3:00 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 9:08 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 6:04 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 6:08 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 6:41 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 6:26 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 7:21 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 7:01 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 8:02 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 7:17 pm)
Re: Back to the future., Pavel Machek, (Thu May 3, 11:25 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 7:45 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 7:59 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 8:18 pm)
Re: Back to the future., Pavel Machek, (Sat May 5, 7:42 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 9:00 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 9:12 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 8:54 pm)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 9:44 pm)
Re: Back to the future., Pavel Machek, (Sat Apr 28, 4:50 am)
Re: Back to the future., David Lang, (Sat Apr 28, 2:32 pm)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 3:14 pm)
Re: Back to the future., David Lang, (Sat Apr 28, 2:44 pm)
Re: Back to the future., Linus Torvalds, (Sat Apr 28, 12:28 pm)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 1:50 pm)
Re: Back to the future., Linus Torvalds, (Sat Apr 28, 5:25 pm)
Re: Back to the future., Pavel Machek, (Sun Apr 29, 4:23 am)
Re: Back to the future., Rafael J. Wysocki, (Sun Apr 29, 5:22 am)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 7:03 pm)
Re: Back to the future., Linus Torvalds, (Sat Apr 28, 7:45 pm)
Re: Back to the future., Kyle Moffett, (Sat Apr 28, 11:43 pm)
Re: Back to the future., Rafael J. Wysocki, (Sun Apr 29, 4:57 am)
Re: Back to the future., Pavel Machek, (Sun Apr 29, 4:59 am)
Re: Back to the future., Rafael J. Wysocki, (Sun Apr 29, 5:32 am)
Re: Back to the future., Nigel Cunningham, (Sat Apr 28, 8:01 pm)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 5:24 am)
Re: Back to the future., Daniel Hazelton, (Fri Apr 27, 10:51 pm)
progress meter in s2disk (was Re: Back to the future.), Pavel Machek, (Sat Apr 28, 3:00 am)
Re: Back to the future., Paul Mackerras, (Fri Apr 27, 8:50 pm)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 7:57 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 7:50 pm)
Re: Back to the future., Linus Torvalds, (Fri Apr 27, 8:40 pm)
Re: Back to the future., Pavel Machek, (Thu May 3, 1:18 pm)
Re: Back to the future., David Lang, (Sun May 6, 10:13 pm)
Re: Back to the future., Kyle Moffett, (Sun May 6, 11:33 pm)
Re: Back to the future., Pavel Machek, (Mon May 7, 8:48 am)
Re: Back to the future., Oliver Neukum, (Mon May 7, 8:52 am)
Re: Back to the future., , (Mon May 7, 10:37 am)
Re: Back to the future., Pavel Machek, (Mon May 7, 3:51 pm)
Re: Back to the future., , (Mon May 7, 3:55 pm)
Re: Back to the future., Pavel Machek, (Mon May 7, 4:38 pm)
Re: Back to the future., Disconnect, (Tue May 8, 1:36 pm)
Re: Back to the future., Oliver Neukum, (Sat Apr 28, 2:58 am)
Re: Back to the future., Pekka J Enberg, (Sat Apr 28, 5:16 am)
Re: Back to the future., David Lang, (Sat Apr 28, 2:28 pm)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 6:07 pm)
Re: Back to the future., Kyle Moffett, (Fri Apr 27, 9:03 pm)
Re: Back to the future., Pavel Machek, (Thu May 3, 11:10 am)
Re: Back to the future., Kyle Moffett, (Thu May 3, 12:53 pm)
Re: Back to the future., David Greaves, (Fri May 4, 3:52 am)
Re: Back to the future., Kyle Moffett, (Fri May 4, 9:27 am)
Re: Back to the future., Rafael J. Wysocki, (Fri Apr 27, 9:15 pm)
Re: Back to the future., David Lang, (Fri Apr 27, 8:51 pm)
Re: Back to the future., Kyle Moffett, (Fri Apr 27, 9:25 pm)
Re: Back to the future., Oliver Neukum, (Fri Apr 27, 5:50 am)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 6:12 am)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 6:35 am)
Re: Back to the future., David Lang, (Sat Apr 28, 2:43 pm)
Re: Back to the future., Rafael J. Wysocki, (Sat Apr 28, 3:37 pm)
Re: Back to the future., Oliver Neukum, (Fri Apr 27, 3:07 pm)
Re: Back to the future., Pekka Enberg, (Sat Apr 28, 5:22 am)
Re: Back to the future., Oliver Neukum, (Sat Apr 28, 9:37 am)
Re: Back to the future., Pavel Machek, (Thu May 3, 8:06 am)
Re: Back to the future., Indan Zupancic, (Fri May 4, 5:52 pm)
Re: Back to the future., Pavel Machek, (Sat May 5, 5:16 am)
Re: Back to the future., Indan Zupancic, (Sat May 5, 8:02 am)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 2:34 am)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 2:50 am)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 3:03 am)
Re: Back to the future., Pekka J Enberg, (Fri Apr 27, 3:24 am)
Re: Back to the future., Bill Davidsen, (Sat Apr 28, 3:09 pm)
Re: Back to the future., Xavier Bestel, (Thu Apr 26, 1:03 pm)
Re: Back to the future., Linus Torvalds, (Thu Apr 26, 1:34 pm)
Re: Back to the future., Pekka Enberg, (Fri Apr 27, 3:51 am)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 4:08 pm)
Re: Back to the future., Linus Torvalds, (Thu Apr 26, 4:45 pm)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 4:50 pm)
Re: Back to the future., Olivier Galibert, (Thu Apr 26, 8:10 pm)
Re: Back to the future., Daniel Pittman, (Fri Apr 27, 6:21 am)
Re: Back to the future., Nigel Cunningham, (Fri Apr 27, 7:19 pm)
Re: Back to the future., Rafael J. Wysocki, (Thu Apr 26, 6:08 pm)
Re: Back to the future., Linus Torvalds, (Thu Apr 26, 7:15 pm)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 6:20 pm)
Re: Back to the future., Theodore Tso, (Thu Apr 26, 5:38 pm)
Re: Back to the future., Christoph Hellwig, (Fri Apr 27, 6:10 am)
Re: Back to the future., Pekka Enberg, (Thu Apr 26, 4:17 am)
Re: Back to the future., Nigel Cunningham, (Thu Apr 26, 5:28 am)
Re: Back to the future., Luca Tettamanti, (Thu Apr 26, 1:29 pm)