On Wed, Jan 02, 2013 at 08:45:22PM -0500, Bradley C. Kuszmaul wrote: > Thanks for the help. I got results similar to yours. However, the > hole punching is much faster if you create the file with fallocate > than if you actually write some data into it. > fallocate and then hole-punch is about 1us per hole punch. > write and then hole-punch is about 90us per hole punch. No surprise - after a write the hole punch has a lot more to do. I modified the test program to not use O_TRUNC, then ran: $ /usr/sbin/xfs_io -f -c "truncate 0" -c "pwrite -b 1m 0 20g" /mnt/scratch/blah wrote 21474836480/21474836480 bytes at offset 0 20.000 GiB, 20480 ops; 0:00:30.00 (675.049 MiB/sec and 675.0491 ops/sec) $ sync $ time ./a.out real 0m1.664s user 0m0.000s sys 0m1.656s $ Why? perf top indicates that pretty quickly: 12.80% [kernel] [k] free_hot_cold_page 10.62% [kernel] [k] block_invalidatepage 10.62% [kernel] [k] _raw_spin_unlock_irq 8.35% [kernel] [k] kmem_cache_free 6.07% [kernel] [k] _raw_spin_unlock_irqrestore 3.65% [kernel] [k] put_page 3.51% [kernel] [k] __wake_up_bit 3.27% [kernel] [k] find_get_pages 2.84% [kernel] [k] get_pageblock_flags_group 2.66% [kernel] [k] cancel_dirty_page 2.09% [kernel] [k] truncate_inode_pages_range The page cache has to have holes punched in it after the write. So, lets rule that out by discarding it separately, and see just what the extent manipulation overhead is: $ rm -f /mnt/scratch/blah $ /usr/sbin/xfs_io -f -c "truncate 0" -c "pwrite -b 1m 0 20g" /mnt/scratch/blah wrote 21474836480/21474836480 bytes at offset 0 20.000 GiB, 20480 ops; 0:00:27.00 (749.381 MiB/sec and 749.3807 ops/sec) $ sudo sh -c "echo 3 > /proc/sys/vm/drop_caches" $ time ./a.out real 0m0.347s user 0m0.000s sys 0m0.332s $ Which is the same as the fallocate/punch method gives.... > But 90us is likely to be plenty fast, so it's looking good. ( I'll > try to track down why my other program was slow.) If you open the file O_SYNC or O_DSYNC, then you'll still get synchronous behaviour.... Cheers, Dave. -- Dave Chinner david@xxxxxxxxxxxxx _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
