On Mon, Feb 11, 2013 at 04:05:01PM +1100, Dave Chinner wrote: > From: Dave Chinner <dchinner@xxxxxxxxxx> > > Speculative preallocation based on the current file size works well > for contiguous files, but is sub-optimal for sparse files where the > EOF preallocation can fill holes and result in large amounts of > zeros being written when it is not necessary. > > The algorithm is modified to prevent EOF speculative preallocation > from triggering larger allocations on IO patterns of > truncate--to-zero-seek-write-seek-write-.... which results in > non-sparse files for large files. This, unfortunately, is the way cp > now behaves when copying sparse files and so needs to be fixed. > > What this code does is that it looks at the existing extent adjacent > to the current EOF and if it determines that it is a hole we disable > speculative preallocation altogether. To avoid the next write from > doing a large prealloc, it takes the size of subsequent > preallocations from the current size of the existing EOF extent. > IOWs, if you leave a hole in the file, it resets preallocation > behaviour to the same as if it was a zero size file. > > Example new behaviour: > > $ xfs_io -f -c "pwrite 0 31m" \ > -c "pwrite 33m 1m" \ > -c "pwrite 128m 1m" \ > -c "fiemap -v" /mnt/scratch/blah > wrote 32505856/32505856 bytes at offset 0 > 31 MiB, 7936 ops; 0.0000 sec (1.608 GiB/sec and 421432.7439 ops/sec) > wrote 1048576/1048576 bytes at offset 34603008 > 1 MiB, 256 ops; 0.0000 sec (1.462 GiB/sec and 383233.5329 ops/sec) > wrote 1048576/1048576 bytes at offset 134217728 > 1 MiB, 256 ops; 0.0000 sec (1.719 GiB/sec and 450704.2254 ops/sec) > /mnt/scratch/blah: > EXT: FILE-OFFSET BLOCK-RANGE TOTAL FLAGS > 0: [0..65535]: 96..65631 65536 0x0 > 1: [65536..67583]: hole 2048 > 2: [67584..69631]: 67680..69727 2048 0x0 > 3: [69632..262143]: hole 192512 > 4: [262144..264191]: 262240..264287 2048 0x1 > > Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx> Applied. Regards, Ben _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
