On Sun, 30 May 1999 06:32:00 +0100, Steve Dodd <dirk@loth.demon.co.uk>
said:
> I understand there's a difference between journalling and logging;
The two terms are both used so interchangeably that there isn't any
point in trying to make such a distinction.
> guessing wildly, I'd say journalling is what you described, whereas
> logging is a system which stores incremental changes to filesystem
> state with 'last known good' checkpoints happening periodically.
That is what journaling is. It just depends on exacly what
consistency semantics those checkpoints have: metadata consistency or
full data consistency. In general, true data consistency can only be
achieved if the application is involved in providing transactional
updates, so unless the application is doing synchronous data writes,
there isn't much point in trying to provide this.
> So in the latter case reading would indeed be slower as you'd have
> to look at all the changes that had occurred since the last
> checkpoint.
Not really, it is easy to avoid that cost penalty, especially since
most of the recently-journaled data will be in cache anyway (and many
journaling schemes actually enforce that property).
--Stephen
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