Re: Why does btrfs benchmark so badly in this case?

[Zach Brown <zab@redhat.com>]

> I also don't know if any common use fs has an optimization whereby
> just the modified sector(s) is overwritten, rather than all sectors
> making up the file system block being modified.

Most of them do.  The generic direct io path allows sector sized dio.
The very first bit of do_blockdev_direct_IO() is testing first for file
system block size alignment then for block device sector size alignment.

You can see this easily with dd conv=notrunc oflags=direct and blktrace.

# blockdev --getss /dev/sda
512
# blockdev --getbsz /dev/sda
4096

# blktrace -d /dev/sda -a issue -o - | blkparse -i - &

$ dd if=/dev/zero of=file bs=4096 count=1 oflag=direct conv=notrunc
  8,0    3       14    35.957320002 17941  D  WS 137297704 + 8 [dd]

$ dd if=/dev/zero of=file bs=512 count=1 oflag=direct conv=notrunc
  8,0    1        4    31.405641362 17940  D  WS 137297704 + 1 [dd]

- z