Date: Tue, 07 Jun 2005 22:25:58 +0100 From: David Malone <dwmalone@maths.tcd.ie> To: Scott Long <scottl@samsco.org> Cc: scottl@FreeBSD.org, hackers@FreeBSD.org, Pawel Jakub Dawidek <pjd@FreeBSD.org>, phk@FreeBSD.org, Ivan Voras <ivoras@fer.hr> Subject: Re: Google SoC idea Message-ID: <200506072225.aa16681@salmon.maths.tcd.ie> In-Reply-To: Your message of "Tue, 07 Jun 2005 14:52:44 MDT." <42A6091C.40409@samsco.org>
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> The problem with journalling at the block layer is that you pretty much > become forced to journal metadata and data, since the block layer really > doesn't know the distinction, Definitely - I guess I should have stated that explicitly. > Full journalling has many drawbacks from the viewpoint of > speed and complexity, of course. So you really want to be able to do > just metadata journalling. The complexity of full journaling (for filesystems that offer a consistent version of themselves to the disk layer) would not seem to be that high: a scheme like Ivan's seems to achieve it. Maybe I've missed something? For us, journaling the metadata is of interest because we're interested in avoiding fscks. I suppose full journaling may be of interest to people with other applications, despite the unfortunate performance in many situations. > An alternate SoC project that would be very useful is block-level > snapshots. I'm not sure if I'll be able to retain the filesystem > snapshot functionality in UFS with journalling enabled, so moving to > doing the snapshots in the block layer would be a good way to make up > for this. Beware that while the GEOM transform would be pretty > straight-forward to write, the real trick comes from being able to make > the consumer of a block device (a filesystem, maybe) flush itself to a > consistent state while the snapshot is being taken. The infrastructure > for this is the part that is very interesting, but also the most work. Definitely another interesting project, though maybe a bit too much work to ask someone to do for $4500 over a summer... David.
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