On Tue, Nov 08, 2016 at 05:39:31PM -0500, Brian Foster wrote: > 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. I think it definitely has value for preventing COW overwrite fragmentation - this will be an issue if people start reflinking files widely (e.g. container roots) and then occasionally overwriting files completely. > FWIW, that COW fork fiemap hack I sent a bit ago came in handy for > playing with this as well. :) It might be worth keeping these two patchsets together for the purposes of development and review. The fiemap hack by itself is neat, but having a demonstrated use for development of new features makes it more than just a "neat hack". :P Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx -- 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
