Re: integrity question

[Chris Mason <mason@suse.com>]

On Mon, 2002-05-27 at 22:26, Jean-Francois Landry wrote:
> On Mon, May 27, 2002 at 10:35:24AM +0100, Toby Dickenson wrote:
> > On Sunday 26 May 2002 6:25 am, Jean-Francois Landry wrote:
> > 
> > thanks for your time,
> > 
> > >> 1. write to A/B/somefileX and fsync it
> > >> 2. mkdir A/C
> > >> 3. rename A/B/somefile to A/C/somefile
> > >> 4. rmdir A/B
> > >> 5. power loss
> > >>
> > >> I would like to guarantee that, after journal replay, 'somefile' is in
> > >> either of those two directories (or both). Am I correct to think that I
> > >> dont need any other syncs in there?
> > >
> > >You are correct, renames are atomic on journalling filesystems.
> > >So, no problems with half-written directory entries, if you lose power
> > >at the wrong time the journal replay procedure will throw the
> > >transaction out and you end up as if you never issued a rename at all.
> > 
> > Atomic rename isnt quite enough in the scenario I described. The filesystem 
> > also needs to take care that the step 3 is not committed before step 2, and 
> > that step 4 is not committed before step 3.
> 
> AFAIK, all transactions are written to the journal in the order they
> were issued, so this problem won't arise.

Correct.  Only data blocks can be written out of order, but the fsync
protects you from that.

-chris