Re: Back to the future.

Pavel Machek
2007-04-26T22:40:32Z

Hi!

> > * 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)

This is basically how uswsusp is designed. (We do not use system call,
you just read from /dev/snapshot, and you have to make few ioctls to
stop the other tasks).

> 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.

Which is what I did long time ago, during uswsusp development.

> 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.

Well... We decided not to do this in the fully working system. SIGSTOP
is just not strong enough, and we want the snapshot atomic.

Now, it would be _very_ nice to be able to snapshot system and
continue running, but I just don't see how to do it without extensive
filesystem support.
Pavel

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