Re: [RFC] add FIEMAP ioctl to efficiently map file allocation

On Oct 29, 2007  16:13 -0600, Andreas Dilger wrote:

On Oct 29, 2007  13:57 -0700, Mark Fasheh wrote:

quoted

I'm a little bit confused by fe_offset. Is it a physical offset, or a
logical offset? The reason I ask is that your description above says "FIEMAP
ioctl will return the logical to physical mapping for the extent that
contains the specified logical byte address." Which seems to imply physical,
but your math to get to the next logical start in a very fragmented file,
implies that fe_offset is a logical offset:

       fm_start = fm_extents[fm_extent_count - 1].fe_offset +
                         fm_extents[fm_extent_count - 1].fe_length + 1; 

Note the distinction between "fe_offset" (which is a physical offset for
a single extent) and "fm_offset" (which is a logical offset for that file).

Actually, that is completely bunk.  What it should say is something like:
"filefrag can easily call the FIEMAP ioctls repeatedly using the returned
fm_start and fm_length as the start offset for the next ioctl:

        fiemap.fm_start = fiemap.fm_start + fiemap.fm_length + 1;

Cheers, Andreas
--
Andreas Dilger
Sr. Software Engineer, Lustre Group
Sun Microsystems of Canada, Inc.