On Fri, 29 Dec 2006 18:32:07 -0500 Theodore Tso <tytso@xxxxxxx> wrote: > On Fri, Dec 29, 2006 at 02:42:51PM -0800, Linus Torvalds wrote: > > I think ext3 is terminally crap by now. It still uses buffer heads in > > places where it really really shouldn't, and as a result, things like > > directory accesses are simply slower than they should be. Sadly, I don't > > think ext4 is going to fix any of this, either. > > Not just ext3; ocfs2 is using the jbd layer as well. I think we're > going to have to put this (a rework of jbd2 to use the page cache) on > the ext4 todo list, and work with the ocfs2 folks to try to come up > with something that suits their needs as well. Fortunately we have > this filesystem/storage summit thing coming up in the next few months, > and we can try to get some discussion going on the linux-ext4 mailing > list in the meantime. Unfortunately, I don't think this is going to > be trivial. I suspect it would be insane to move any part of JBD (apart from the ordered-data flush) to use pagecache. The whole thing is fundamentally block-based. But only for metadata - there's no strong reason why ext3/4 needs to manipulate file data via buffer_heads if data=journal and chattr +j aren't in use. We could possibly move ext3/4 directories out of the blockdev pagecache and into per-directory pagecache, but that wouldn't change anything - the journalling would still be block-based. Adam Richter spent considerable time a few years ago trying to make the mpage code go direct-to-BIO in all cases and we eventually gave up. The conceptual layering of page<->blocks<->bio is pretty clean, and it is hard and ugly to fully optimise away the "block" bit in the middle. buffer_heads become more important with large PAGE_CACHE_SIZE. I'd expect nobh mode to be quite inefficient with some workloads on 64k pages. We need that representation of the state (and location) of the block-sized hunks which make up the page. > If we do get this fixed for ext4, one interesting question is whether > people would accept a patch to backport the fixes to ext3, given the > the grief this is causing the page I/O and VM routines. OTOH, reiser3 > probably has the same problems, and I suspect the changes to ext3 to > cause it to avoid buffer heads, especially in order to support for > filesystem blocksizes < pagesize, are going to be sufficiently risky > in terms of introducing regressions to ext3 that they would probably > be rejected on those grounds. So unfortunately, we probably are going > to have to support flushes via buffer heads for the foreseeable > future. We'll see. - To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
