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

[Andreas Dilger <adilger@sun.com>]

On Oct 29, 2007  17:11 -0700, Mark Fasheh wrote:
> On Mon, Oct 29, 2007 at 04:13:02PM -0600, Andreas Dilger wrote:
> > > Btrfs, Ocfs2, and Gfs2 pack small amounts of user data directly in inode
> > > blocks.
> > 
> > Hmm, but part of the issue would be how to request the extra data, and
> > what offset it would be given?  One could, for example, use negative
> > offsets to represent metadata or something, or add a FIEMAP_EXTENT_META
> > or similar, I hadn't given that much thought.
> 
> Well, fe_offset and fe_length are already expressed in bytes, so we could
> just put the byte offset to where the inline data starts in there. fe_length
> is just used as the length allocated for inline-data.
> 
> If fe_offset is required to be block aligned, then we could add a field to
> express an offset within the block where data would be found - say
> 'fe_data_start_offset'. In the non-inline case, we could guarantee that
> fe_data_start_offset is zero. That way software which doesn't want to care
> whether something is inline-data (for example, a backup program) or not
> could just blidly add it to fe_offset before looking at the data.

Oh, I was confused as to what you are asking.  Mapping in-inode data is
just fine using the existing interface.  The byte offset of the data is
given, and the "FIEMAP_EXTENT_NO_DIRECT" flag is set to indicate that it
isn't necessarily safe to do IO directly to that byte offset in the file
(e.g. tail packed, compressed data, etc).

I was thinking you were asking how to map metadata (e.g. indirect blocks).

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