On Sun, Aug 30, 2009 at 12:52:29PM -0400, Christoph Hellwig wrote: > On Sat, Aug 29, 2009 at 10:54:18PM -0400, Theodore Ts'o wrote: > > MAX_WRITEBACK_PAGES was hard-coded to 1024 because of a concern of not > > holding I_SYNC for too long. But this shouldn't be a concern since > > I_LOCK and I_SYNC have been separated. So make it be a tunable and > > change the default to be 32768. > > > > This change is helpful for ext4 since it means we write out large file > > in bigger chunks than just 4 megabytes at a time, so that when we have > > multiple large files in the page cache waiting for writeback, the > > files don't end up getting interleaved. There shouldn't be any downside. > > > > http://bugzilla.kernel.org/show_bug.cgi?id=13930 > > The current writeback sizes are defintively too small, we shoved in > a hack into XFS to bump up nr_to_write to four times the value the > VM sends us to be able to saturate medium sized RAID arrays in XFS. Hmm, should we make it be a per-superblock tunable so that it can either be tuned on a per-block device basis or the filesystem code can adjust it to their liking? I thought about it, but decided maybe it was better to keeping it simple. > Turns out this was not enough and at least for Chris Masons array > we only started seaturating at * 16. I suspect you patch will give > a similar effect. So you think 16384 would be a better default? The reason why I picked 32768 was because that was the size of the ext4 block group, but it was otherwise it was totally arbitrary. I haven't done any benchmarking yet, which is one of the reasons why I thought about making it a tunable. > And btw, I think referring to the historic code in the comment is not > a good idea, it's just going to ocnfuse the heck out of everyone looking > at it in the future. The information above makes sense for the commit > message. Yeah, good point. > And the other big question is how this interacts with Jens' new per-bdi > flushing code that we still hope to merge in 2.6.32. Jens? What do you think? Fixing MAX_WRITEBACK_PAGES was something I really wanted to merge in 2.6.32 since it makes a huge difference for the block allocation layout for a "rsync -avH /old-fs /new-fs" when we are copying bunch of large files (say, 800 meg iso images) and so the fact that the writeback routine is writing out 4 megs at a time, means that our files get horribly interleaved and thus get fragmented. I initially thought about adding some massive workarounds in the filesystem layer (which is I guess what XFS did), but I ultimately decided this was begging to be solved in the page writeback code, especially since it's *such* an easy fix. > Maybe we'll actually get some sane writeback code for the first time. To quote from "Fiddler on the Roof", from your lips to God's ears.... :-) - Ted -- 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
