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. 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
