Here's an observation that is not a bug (as in data corruption), just somewhat odd and unnecessary behaviour. It could be considered a performance or scalability bug.
I've noticed that slow slow buffered writes create a huge number of unnecessary 4k sized extents. At first I wrote it off as odd buffering behaviour of the application (a download manager), but it can be easily reproduced. For example: holger>wget --limit-rate=1m https://cdn.kernel.org/pub/linux/kernel/v4.x/testing/linux-4.5-rc6.tar.xz (..downloads with 1 MB/s..) holger>sync holger>filefrag -ek linux-4.5-rc6.tar.xz Filesystem type is: 9123683e File size of linux-4.5-rc6.tar.xz is 88362576 (86292 blocks of 1024 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 14219: 230807476.. 230821695: 14220: 1: 14220.. 14223: 230838148.. 230838151: 4: 230821696: 2: 14224.. 29215: 230822324.. 230837315: 14992: 230838152: 3: 29216.. 44207: 230838152.. 230853143: 14992: 230837316: 4: 44208.. 44211: 230869576.. 230869579: 4: 230853144: 5: 44212.. 59199: 230853968.. 230868955: 14988: 230869580: 6: 59200.. 74191: 230869588.. 230884579: 14992: 230868956: 7: 74192.. 74195: 230898332.. 230898335: 4: 230884580: 8: 74196.. 86291: 230885620.. 230897715: 12096: 230898336: last,eof linux-4.5-rc6.tar.xz: 9 extents found Slower writes will generate even more extents; another ~200MB file had >900 extents. As expected defragment will collapse these stray extents with their successors: holger>btrfs filesystem defragment linux-4.5-rc6.tar.xz holger>sync holger>filefrag -ek linux-4.5-rc6.tar.xz Filesystem type is: 9123683e File size of linux-4.5-rc6.tar.xz is 88362576 (86292 blocks of 1024 bytes) ext: logical_offset: physical_offset: length: expected: flags: 0: 0.. 14219: 230807476.. 230821695: 14220: 1: 14220.. 29215: 230922128.. 230937123: 14996: 230821696: 2: 29216.. 44207: 230838152.. 230853143: 14992: 230937124: 3: 44208.. 59199: 230937124.. 230952115: 14992: 230853144: 4: 59200.. 74191: 230869588.. 230884579: 14992: 230952116: 5: 74192.. 86291: 230952116.. 230964215: 12100: 230884580: last,eof linux-4.5-rc6.tar.xz: 6 extents found The obviously page-sized 4k extents happen to coincide with the 30s tx commit (2 * ~15 MB at 1 MB/s). It looks like a benign race, as if the last dirty page gets special treatment instead of being merged into wherever it should go. That just seems wasteful to me. Anyone got an idea? -h -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
