Re: Back to the future.

[Linus Torvalds <torvalds@linux-foundation.org>]

On Thu, 26 Apr 2007, Nigel Cunningham wrote:
> 
> * Doing things in the right order? (Prepare the image, then do the
> atomic copy, then save).

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.

> * Mulithreaded I/O (might as well use multiple cores to compress the
> image, now that we're hotplugging later).
> * Support for > 1 swap device.
> * Support for ordinary files.
> * Full image option.
> * Modular design?

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