Re: Back to the future.

Pekka J Enberg
2007-04-27T06:50:47Z

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> Sorry Pekka, but that's just broken.

It certainly isn't.

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> It implies firstly that we tell all userspace programs "I'm sorry, but
> I'm suspending at the moment. Can you tip toe quietly around while I do
> it?" You can't seriously expect every userspace program to be modified
> to adjust it's behaviour according to whether we're writing a snapshot
> to disk at the moment or not.

You don't need to modify other programs. You just need to display the
progress bar and block _user input_. I don't even claim to know X, but I
would be extremely surprised if you technically can't say "don't let
the user touch any other windows except this one." The user couldn't care
less whether tasks are frozen or not by the kernel. What matters is that
the user can't shoot himself in the foot while snapshotting.

Furthermore, we probably do need to do other things to ensure safety, like
remounting filesystems read-only but again, this has nothing to do with
snapshotting per se. What the kernel needs to worry about is (1) providing
an atomic snapshot that is consistent and (2) resuming to that snapshot
safely. If the _user_ loses data that was generated between snapshot +
shutdown, it's absolutely no concern for the snapshot operation!

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> It also implies that we can prepare a snapshot and then happily have the
> contents of the disk change so that they don't match the superblock and
> other filesystem details we just saved in the snapshot. We can't. At
> least not without modifying all the filesystems so that (at a minimum)
> they know how to throw away all the metadata they have at resume time
> and reread it from disk.

But you just explained how we can! We shouldn't bend over backwards for
snapshotting just because the filesystems don't currently support
something we need.

On Fri, 27 Apr 2007, Nigel Cunningham wrote:
> By the way, sorry. This email feels like it is pouring a lot of cold
> water on your ideas. I don't want to be negative!

Don't worry, I am used to cold water :-).

Pekka