Re: ext3_ordered_writepage() questions

[Jamie Lokier <jamie@shareable.org>]

Theodore Ts'o wrote:
> If the application cares about the precise ordering of data blocks
> being written out with respect to the mtime field, I'd respectfully
> suggest that the application use data journalling mode --- and note
> that most applications which update existing data blocks, especially
> relational databases, either don't care about mtime, have their own
> data recovering subsystems, or both.

I think if you're thinking this only affects "applications" or
individual programs (like databases), then you didn't think about the
example I gave.

Scenario:

   - Person has two computers, A and B.
     Maybe a desktop and laptop.  Maybe office and home machines.

   - Sometimes they do work on A, sometimes they do work on B.
     Things like editing pictures or spreadsheets or whatever.

   - They use "rsync" to copy their working directory from A to B, or
     B to A, when they move between computers.

   - They're working on A one day, and there's a power cut.

   - Power comes back.

   - They decide to start again on A, using "rsync" to copy from B to A
     to get a good set of files.

   - "rsync" is believed to mirror directories from one place to
     another without problems.  It's always worked for them before.
     (Heck, until this thread came up, I assumed it would always work).

   - ext3 is generally trusted, so no fsck or anything else special is
     thought to be required after a power cut.

   - So after running "rsync", they believe it's safe to work on A.

     This assumption is invalid, because of ext3's data vs. mtime
     write ordering when they were working on A before the power cut.

     But the user doesn't expect this.  It's far from obvious (except
     to a very thoughtful techie) that rsync, which always works
     normally and even tidies up mistakes normally, won't give correct
     results this time.

   - So they carry on working, with corrupted data.  Maybe they won't
     notice for a long time, and the corruption stays in their work
     project.

No individual program or mount option is at fault in the above
scenario.  The combination together creates a fault, but only after a
power cut.  The usage is fine in normal use and for all other typical
errors which affect files.

Technically, using data=journal, or --checksum with rsync, would be fine.

But nobody _expects_ to have to do that.  It's a surprise.

And they both imply a big performance overhead, so nobody is ever
advised to do that just to be safe for "ordinary" day to day work.

-- Jamie
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