On Fri, Sep 21, 2012 at 05:03:41AM +0800, ching wrote:

AFAIK, the autodefrag will read related extents and mark them dirty,
io niceness should be applicable to the read operation

I was hesitant to believe that, but tried it and it's right:

$ dd if=/dev/zero bs=4k count=1 seek=15 of=file
$ dd if=/dev/zero bs=4k count=1 seek=13 of=file conv=notrunc
$ dd if=/dev/zero bs=4k count=1 seek=14 of=file conv=notrunc

Filesystem type is: 9123683e
File size of file is 65536 (16 blocks, blocksize 4096)
 ext logical physical expected length flags
   0      13     1027               1
   1      14     1050     1027      1
   2      15     1025     1050      1 eof
file: 4 extents found

remount with autodefrag:

$ dd if=/dev/zero bs=4k count=1 seek=16 of=file conv=notrunc

Filesystem type is: 9123683e
File size of file is 69632 (17 blocks, blocksize 4096)
 ext logical physical expected length flags
   0      13     1033               4 eof
file: 2 extents found


david