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--- Comment #152 from Karl Tomlinson (:karlt) <mozbugz@karlt.net>  2008-05-25 22:12:23 PDT ---

Created an attachment (id=322475)
 --> (https://bugzilla.mozilla.org/attachment.cgi?id=322475)
fdatasync/sync_file_range test program

fdatasync/sync_file_range test program

This first creates a file of length 1 then does one fsync on the new file.
Then the file is continually modified without changing the length and synced
after each modification using one of three methods (somewhat randomly
selected): fsync/fdatasync/sync_file_range.

The I/O load for the test results below was produced using dd with a small
blocksize to limit the I/O some:

dd if=/dev/zero of=large bs=64 count=$((3*1024*1024*1024/64))

I used ltrace instead of strace as my strace didn't find sync_file_range (and
my glibc-2.5 libraries don't seem to have a sync_file_range function), so
sync_file_range appears below as "syscall(277".

rm -f datasync-test.tmp &&
ltrace -t -T -e trace=,fsync,fdatasync,syscall ./a.out

16:12:59 fsync(3) = 0                   <11.864858>
16:13:13 fdatasync(3) = 0               <14.706356>
16:13:30 fsync(3) = 0                   <12.832373>
16:13:45 syscall(277, 3, 0, 1, 7) = 0   <0.343116>
16:13:49 fdatasync(3) = 0               <8.231468>
16:14:01 syscall(277, 3, 0, 1, 7) = 0   <2.347144>
16:14:06 fsync(3) = 0                   <6.938656>
16:14:16 fdatasync(3) = 0               <8.359644>
16:14:27 fsync(3) = 0                   <5.928242>
16:14:35 syscall(277, 3, 0, 1, 7) = 0   <0.009531>
16:14:39 fdatasync(3) = 0               <7.356126>
16:14:50 fsync(3) = 0                   <6.402128>
16:14:59 syscall(277, 3, 0, 1, 7) = 0   <0.802706>
16:15:03 syscall(277, 3, 0, 1, 7) = 0   <2.985404>
16:15:08 fsync(3) = 0                   <4.722020>
16:15:15 fdatasync(3) = 0               <6.532945>
16:15:24 fdatasync(3) = 0               <2.294488>
16:15:30 fsync(3) = 0                   <7.986250>
16:15:40 syscall(277, 3, 0, 1, 7) = 0   <1.409809>
16:15:45 fdatasync(3) = 0               <5.404190>

The results are consistent with fdatasync being implemented as fsync on ext3.

They show the potential for considerable savings from growing (and shrinking)
files in large hunks and using sync_file_range (which also should reduce the
impact on the rest of the filesystem).