On Wed, Aug 29, 2007 at 02:33:08AM +1000, David Chinner wrote: > On Tue, Aug 28, 2007 at 11:08:20AM -0400, Chris Mason wrote: > > On Wed, 29 Aug 2007 00:55:30 +1000 > > David Chinner <dgc@xxxxxxx> wrote: > > > On Fri, Aug 24, 2007 at 09:55:04PM +0800, Fengguang Wu wrote: > > > > On Thu, Aug 23, 2007 at 12:33:06PM +1000, David Chinner wrote: > > > > > On Wed, Aug 22, 2007 at 09:18:41AM +0800, Fengguang Wu wrote: > > > > > > On Tue, Aug 21, 2007 at 08:23:14PM -0400, Chris Mason wrote: > > > > > > Notes: > > > > > > (1) I'm not sure inode number is correlated to disk location in > > > > > > filesystems other than ext2/3/4. Or parent dir? > > > > > > > > > > The correspond to the exact location on disk on XFS. But, XFS has > > > > > it's own inode clustering (see xfs_iflush) and it can't be moved > > > > > up into the generic layers because of locking and integration into > > > > > the transaction subsystem. > > > > > > > > > > > (2) It duplicates some function of elevators. Why is it > > > > > > necessary? > > > > > > > > > > The elevators have no clue as to how the filesystem might treat > > > > > adjacent inodes. In XFS, inode clustering is a fundamental > > > > > feature of the inode reading and writing and that is something no > > > > > elevator can hope to acheive.... > > > > > > > > Thank you. That explains the linear write curve(perfect!) in Chris' > > > > graph. > > > > > > > > I wonder if XFS can benefit any more from the general writeback > > > > clustering. How large would be a typical XFS cluster? > > > > > > Depends on inode size. typically they are 8k in size, so anything > > > from 4-32 inodes. The inode writeback clustering is pretty tightly > > > integrated into the transaction subsystem and has some intricate > > > locking, so it's not likely to be easy (or perhaps even possible) to > > > make it more generic. > > > > When I talked to hch about this, he said the order file data pages got > > written in XFS was still dictated by the order the higher layers sent > > things down. > > Sure, that's file data. I was talking about the inode writeback, not the > data writeback. > > > Shouldn't the clustering still help to have delalloc done > > in inode order instead of in whatever random order pdflush sends things > > down now? > > Depends on how things are being allocated. if you've got inode32 allocation > and >1TB filesytsem, then data is nowhere near the inodes. If you've got large > allocation groups, then data is typically nowhere near the inodes, either. If > you've got full AGs, data will be nowehere near the inodes. If you've got > large files and lots of data to write, then clustering multiple files together > for writing is not needed. So in many cases, clustering delalloc writes by > inode number doesn't provide any better I/o patterns than not clustering... > > The only difference we may see is that if we flush all the data on inodes > in a single cluster, we can get away with a single inode cluster write > for all of the inodes.... So we end up with two major cases: - small files: inode and its data are expected to be close enough, hence it can help I_DIRTY_SYNC and/or I_DIRTY_PAGES - big files: inode and its data may or may not be separated - I_DIRTY_SYNC: could be improved - I_DIRTY_PAGES: no better, no worse(it's big I/O, the seek cost is not relevant in any case) Conclusion: _inode_ writeback clustering is enough. Isn't it simple? ;-) Fengguang - To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html