Eric Anderson wrote:
Scott Long wrote:
Richard Coleman wrote:
Scott Long wrote:
/me jumps up and down and waves his hands
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, and definitely not in a
filesystem-independent way (yes, UFS does evil things to the buffer
cache by representing metadata with negative block numbers, but that
is just UFS). 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.
Another hard part of distinguishing between metadata and data is
that filesystems have a habit of migrating disk blocks from holding
metadata to holding data, and vice versa (think indirect pointer
blocks, not inode blocks). If you are only replaying metadata, you
want to make sure that you don't smash data blocks with old metadata.
Coming up with a filesystem independent way to represent all of this
for the block layer is not easy. Filesystems would have to be able
to be modified to provide proper metadata vs. data hints to the
block layer. And if you're going to do that, then why not just make
it a library in VFS, like what Darwin does?
The UFS Journalling work is already well underway, and I expect it
to follow the path of being a VFS library. Note that I'm saying
'library' here, not 'layer'. There really is no way to make
journalling work with an arbitrary filesystem 'for free', whether as
a VFS layer or a GEOM transform, since journalling is 100% dependent
on the filesystem working with the buffer-cache to do sane
operations in a defined in order.
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.
Scott
Scott,
Have you looked at the journaling layer that Matt has been adding to
DragonflyBSD? What you are talking about appears very similar. Or
am I misunderstanding something?
Richard Coleman
[EMAIL PROTECTED]
Ah, you might have misunderstood my use of the term 'VFS library'. This
is distinctly different from a 'VFS layer', which is what Matt did.
I've looked extensively at his work, but unfortunately it doesn't solve
the kinds of problems that I'm looking to solve. After discussing
journalling this evening with the author of BeFS and HFS+J, I'm pretty
happy that I'm taking the approach that I am.
Maybe a good SoC project (but maybe too much work) would be getting the
clustering UFS stuff going.. :)
Eric
THat is more along the lines of a good master's of PhD topic.
Scott
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