Re: [RFC, PATCH 0/6] ext3: do not modify data on-disk when mounting read-only filesystem

Duane Griffin
2008-03-13T12:35:54Z

On 13/03/2008, Daniel Phillips <phillips@phunq.net> wrote:
> Hi Duane,
>
> Thanks for doing this. Some perhaps not so obvious fallout from the bad
> old way of doing things is that ddnap (zumastor) hits an issue in
> replication. Since ddsnap allows journal replay on the downstream
> server and also needs to have an unaltered snapshot to apply deltas
> against, if we do not take special care, Ext3 will come along and
> modify the downstream snapshot even when told not to. Our solution:
> take two snapshots per replication cycle (pretty cheap) so that one can
> be clean and the other can be stepped on at will by the journal replay.
> Ugh.

Ah, good to know, thanks. It looks like you folks are doing some
interesting things there.

> With your hack, we can eventually drop the double snapshot, provided no
> other filesystem is similarly badly behaved.

Excellent, I'm really pleased to hear that.

> Re your page translation table: we already have a page translation
> table, it is called the page cache. If you could figure out which file
> (or metadata) each journal block belongs to, you could just load the
> page table pages back in and presto, done. No need to replay the
> journal at all, you are already back to journal+disk = consistent
> state.

Hmm, interesting. I'll have to have a think about that, thanks.

> mapping. As it stands your solution seems well built, after a quick
> readthrough. Nice looking code. I think you added about 250 lines
> overall, so tight too. Thanks again.

Thanks very much, I appreciate it!

Cheers,
Duane.

--
"I never could learn to drink that blood and call it wine" - Bob Dylan