Re: precise characterization of ext3 atomicity

[Andrew Morton]

Daniel Phillips <phillips@xxxxxxxx> wrote:
>
> On Thursday 04 September 2003 17:55, Andrew Morton wrote:
> > Hans Reiser <reiser@xxxxxxxxxxx> wrote:
> > > Is it correct to say of ext3 that it guarantees and only guarantees
> > > atomicity of writes that do not cross page boundaries?
> >
> > Yes.
> 
> Is that just happenstance, or does Posix or similar mandate it?

Happenstance.

It's semi-trivial to do this in ext3.  You'd open the file with O_ATOMIC
and a write() would either be completely atomic or would return -EFOO
without having written anything.

The thing which prevents this is the ranking order between journal_start()
and lock_page().

It's not trivial but also not too hard to change things so that
journal_start() can rank outside lock_page() - this would also offer some
CPU savings.

Can't say that I'm terribly motivated about the feature though.

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