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Re: How safe is journalling?



On Wed, 31 Oct 2001, Andreas Dilger wrote:

> On Oct 31, 2001  17:42 +0000, Jeremy Sanders wrote:
> > The question is how safe is journalling, in general? The "ordered" mount
> > option for ext3 seems pretty safe, but is it possible that if the drive's
> > power went half way through writing to the journal, could it be left in an
> > inconsistent state? I mean, is writing to the journal an "atomic" process?
>
> This is the whole point of having a journaled filesystem.  The journal
> is only really useful when your system fails.  Otherwise, it is just
> overhead for 99% of uses.
>
> The journal guarantees that the filesystem metadata is always consistent
> after a crash.  You may lose some operations from just before the crash,
> but you probably would have anyways (probably not quite as many as with
> a journal), because of buffering.

That's what I thought, but can the drive to strange things like
re-ordering writes and caching data which may screw up journalling
attempts?

Thanks

Jeremy

-- 
Jeremy Sanders <jss ast cam ac uk>  http://www-xray.ast.cam.ac.uk/~jss/
Pembroke College, Cambridge. UK   Institute of Astronomy, Cambridge. UK





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