On Fri, Sep 25, 2009 at 12:16:19PM +0800, Dave Chinner wrote: > On Thu, Sep 24, 2009 at 10:09:19PM +0800, Wu Fengguang wrote: > > On Thu, Sep 24, 2009 at 09:52:17PM +0800, Arjan van de Ven wrote: > > > On Thu, 24 Sep 2009 21:46:25 +0800 > > > Wu Fengguang <fengguang.wu@xxxxxxxxx> wrote: > > > > > > > > Note that dirty_time may not be unique, so need some workaround. And > > > > the resulted rbtree implementation may not be more efficient than > > > > several list traversals even for a very large list (as long as > > > > superblocks numbers are low). > > > > > > > > The good side is, once sb+dirty_time rbtree is implemented, it should > > > > be trivial to switch the key to sb+inode_number (also may not be > > > > unique), and to do location ordered writeback ;) > > > > > > would you want to sort by dirty time, or by inode number? > > > (assuming inode number is loosely related to location on disk) > > > > Sort by inode number; dirty time will also be considered when judging > > whether the traversed inode is old enough(*) to be eligible for writeback. > > Even if the inode number is directly related to location on disk > (like for XFS), there is no guarantee that the data or related > metadata (indirect blocks) writeback location is in any way related > to the inode number. e.g when using the 32 bit allocator on XFS > (default for > 1TB filesystems), there is _zero correlation_ between > the inode number and the data location. Hence writeback by inode > number will not improve writeback patterns at all. The location ordering is mainly an optimization for _small files_. So no indirect blocks. A good filesystem will put metadata+data as close as possible for small files. Is that true for XFS? > Only the filesystem knows what the best writeback pattern really is; > any change is going to affect filesystems differently. > > > The more detailed algorithm would be: > > > > - put inodes to rbtree with key sb+inode_number > > - in each per-5s writeback, traverse a range of 1/5 rbtree > > - in each traverse, sync inodes that is dirtied more than 5s ago > > > > So the user visible result would be > > - on every 5s, roughly a 1/5 disk area will be visited > > - for each dirtied inode, it will be synced after 5-30s > > Personally, I'd prefer that writeback calls a vector that says > "writeback inodes older than N" and implement something like the > above as the generic mechanism. That way filesystems can override > the generic algorithm if there is a better way to track and write > back dirty inodes for that filesystem. We have wbc->older_than_this. Is it good enough for XFS? Thanks, 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
