On Tue, Nov 08, 2016 at 03:27:32PM -0500, Brian Foster wrote: > Hi all, > > This is an experiment based on an idea for COW fork speculative > preallocation. This is experimental, lightly/barely tested and sent in > RFC form to solicit thoughts, ideas or flames before I spend time taking > it further. > > Patch 1 probably stands on its own. Patches 2 and 3 are some refactoring > and patch 4 implements the basic idea, which is described in the commit > log description. The testing I've done so far is basically similar to > how one would test the effects of traditional speculative preallocation: > write to multiple reflinked files in parallel and examine the resulting > fragmentation. Specifically, I wrote sequentially to 16 different > reflinked files of the same 8GB original (which has two data extents, > completely shared). Without preallocation, the test results in ~248 > extents across the 16 files. With preallocation, the test results in 32 > extents across the 16 files (i.e., 2 extents per file, same as the > source file). > > An obvious tradeoff is the unnecessarily aggressive allocation that > might occur in the event of random writes to a large file (such as in > the cloned VM disk image use case), but my thinking is that the > cowblocks tagging and reclaim infrastructure should manage that > sufficiently (lack of testing notwithstanding). In any event, I'm > interested in any thoughts along the lines of whether this is useful at > all, alternative algorithm ideas, etc. Was about to step out to lunch when this came in, but... Is there an xfstest for this, so I can play too? :) As far as random writes go, some of the reflink tests look at fragmentation behavior. generic/301 generic/302 xfs/180 xfs/182 xfs/184 xfs/192 xfs/193 xfs/198 xfs/200 xfs/204 xfs/208 xfs/208 xfs/211 xfs/215 xfs/218 xfs/219 xfs/221 xfs/223 xfs/224 xfs/225 xfs/226 xfs/228 xfs/230 xfs/231 xfs/232 xfs/344 xfs/345 xfs/346 xfs/347 are the ones that grep 'new extents:' picked up. Will look at the patches when I get back. --D > > Brian > > Brian Foster (4): > xfs: clean up cow fork reservation and tag inodes correctly > xfs: logically separate iomap range from allocation range > xfs: reuse xfs_file_iomap_begin_delay() for cow fork delalloc > xfs: implement basic COW fork speculative preallocation > > fs/xfs/xfs_iomap.c | 132 +++++++++++++++++++++++++++++++++++++++++---------- > fs/xfs/xfs_reflink.c | 28 ++--------- > 2 files changed, 111 insertions(+), 49 deletions(-) > > -- > 2.7.4 > > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
