> 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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