On Tue, Jan 24, 2017 at 05:21:56PM +0100, Christoph Hellwig wrote: > On Tue, Jan 24, 2017 at 11:17:20AM -0500, Brian Foster wrote: > > I thought that while not necessarily guaranteed, generally the entire > > extent gets converted from delalloc to real blocks. > > For buffered I/O that's the case. See the discussion on my recent > xfs_bmapi_write patch. > Yeah, that's the behavior I would have expected... > > IIRC, that's what > > I've seen in the past when looking into the cow fork with bmap. After > > all, isn't that the point of the extent size hint? Allocate wider than > > the write to accommodate potential subsequent writes into a more > > contiguous range. > > Well, for direct I/O that's not what the current code does. Implementing > it might be useful, but I'm not sure how much the front alignment is > going to help in usual worksloads - you'd need a backwards write or > random writes that happen to look almost backwards for it to make > a difference. I suspect most of them time we'd just allocate blocks to > reclaim them again a little later. Hmm, that's not what I'm seeing (not that it really matters, but I'm curious if I'm missing something): # xfs_io -d -c "bmap -v" -c "pwrite 8k 4k" -c "bmap -v" -c "bmap -cv" ./file ./file: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..15]: 80..95 0 (80..95) 16 100000 1: [16..2047]: 160..2191 0 (160..2191) 2032 100000 wrote 4096/4096 bytes at offset 8192 4 KiB, 1 ops; 0.0000 sec (12.440 MiB/sec and 3184.7134 ops/sec) ./file: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..15]: 80..95 0 (80..95) 16 100000 1: [16..23]: 2208..2215 0 (2208..2215) 8 2: [24..2047]: 168..2191 0 (168..2191) 2024 100000 ./file: EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL 0: [0..15]: 2192..2207 0 (2192..2207) 16 1: [16..23]: hole 8 2: [24..255]: 2216..2447 0 (2216..2447) 232 3: [256..2047]: hole 1792 That's on a fairly recent for-next on a fully reflinked file. It looks to me that the cow fork extent is fully allocated immediately (otherwise, I'm not sure what else would have converted it). Brian > -- > 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
