Some journal based FS's will serialize reads when A) there is cached data
(in a buffer cache for example), and there is a requirement for writes to be
committed real time with reads always having the most current data, and B)
when transactioning is implemented above the FS and you have writes and
reads to the same block (do you return the stale data, or do you return the
write that was posted to the journal and cached, but not yet committed?) In
theory, you could avoid serializing on reads, but you may get stale data.
This applies to journalled FS's that post both user data and meta data to
the log file. NTFS, for example, only posts meta data to the log file for
restart, and does not post user writes to the journal. This means you can
lose user data on a restart. If XFS is only writing meta data to the
journal, and not user data, as you suggest, then you are technically correct
that reads do not have to be serialized, however, this also means that XFS
is not a **TRUE** journalling file system because you can lose user data on
restarts, and the benefits it provides for journalling are not much better
than running "fsck" after a system crash.
Surely this is not the case -- this would mean that XFS is no better than
NTFS.
Jeff
----- Original Message -----
From: Jim Mostek <mostek@sgi.com>
To: Andreas Bogk <andreas@andreas.org>
Cc: <jmerkey@timpanogas.com>; <mcai7et2@stud.umist.ac.uk>;
<linux-kernel@vger.rutgers.edu>
Sent: Friday, May 28, 1999 8:58 AM
Subject: Re: XFS and journalling filesystems
> >feature for video: guaranteed bandwidth. Also, journalling slows down
> >reading, but speeds up writing, which is again important for
> >video. So, as long as ext3 is not there, I'll be very happy about XFS.
>
> How does journalling slow down reading? The only time XFS needs
> to create a transaction is when changing meta-data (like a create,
> file extend, remove, ...).
> XFS doesn't do transactions for time updates. Do other FSs?
> XFS doesn't do transactions on writting if the file is already
> allocated.
>
> Jim
>
> >
> >Andreas
> >
> >--
> >Reality is two's complement. See:
> >ftp://ftp.netcom.com/pub/hb/hbaker/hakmem/hacks.html#item154
> >
> >-
> >To unsubscribe from this list: send the line "unsubscribe linux-kernel"
in
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> >Please read the FAQ at http://www.tux.org/lkml/
> >
>
>
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