On Tue, Nov 08, 2016 at 12:48:00PM -0800, Darrick J. Wong wrote: > 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? :) > Not yet.. I've only xfstests tested insofar as it hasn't blown anything up yet. :) Otherwise, I've just run manual write tests to observe whether it is doing what I expect it to in simple cases. It clearly needs more work, as noted in the patch, but if this is something worth pursuing further I can certainly come up with some tests as well. FWIW, that COW fork fiemap hack I sent a bit ago came in handy for playing with this as well. :) > 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. > Ok. > Will look at the patches when I get back. > Thanks! Brian > --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
