Chris Mason writes:
> The only thing reiserfs needs to support this is some way of communicating
> the current process is already in a transaction. *But*, the nesting could
> only be legal if the parent transaction has reserved enough blocks for the
> nested transaction to complete as well. I'll have to check the rules that
> ext3 uses in allowing the nesting.
It is mostly true that ext3 transactions need enough space reserved at the
parent transaction for all nested transactions to complete. However, some
transactions like write and truncate may be split into multiple transactions
if they grow too large. This problem will disappear when ext3 moves to
metadata-only journalling.
> Yes, ext3's jfs and reiserfs have the same fundamental ideas.
> block based write ahead logging. With slight modifications to the jfs,
> wandering logs could even fit in.
What are wandering logs, if I might ask?
> The ext3 jfs might also need small changes to wok cleanly with any
> allocate on flush systems (XFS, reiserfs soon).
Aren't there also changes required to the VFS for allocate-on-flush? I
think this would benefit the performance of all filesystems, but of course
XFS much more than ext2, because XFS was designed from the start to work
with this.
> Very possible, regardless of how you log the data, the api steps need to
> make it all work are almost the same. From what I've read about xfs, it
> could fit into the ext3 jfs as well. I'm assuming GFS will need the most
> complicated journal layer...
It is my understanding also that XFS and ext3 JFS are basically working in
the same way. The real question would be if the XFS developers want to
re-work their code to use ext3 JFS and/or if JFS is sufficiently abstract
to be a plug-in replacement for the XFS journalling code. If it isn't a
proper superset of what XFS does, it would be an indication that JFS needs
a bit of rework. AFAIK, the only real feature that JFS doesn't yet implement
is allowing journalling of multiple filesystems to the same journal, although
I know this is on Stephen's TODO list.
Cheers, Andreas
--
Andreas Dilger \ "If a man ate a pound of pasta and a pound of antipasto,
\ would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/ -- Dogbert
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