Messages in this thread | | | From | "Stephen C. Tweedie" <> | Date | Tue, 1 Jun 1999 00:13:41 +0100 (BST) | Subject | Re: XFS and journalling filesystems |
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Hi,
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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