Re: [RFC][PATCH 0/5] Fiemap, an extent mapping ioctl

[Andreas Dilger <adilger@sun.com>]

On May 27, 2008  13:31 -0700, Joel Becker wrote:
> On Tue, May 27, 2008 at 11:56:22AM -0700, Mark Fasheh wrote:
> > > > If the request has the FIEMAP_FLAG_NUM_EXTENTS flag set, then calling
> > > > this helper is not necessary and fi_extents_mapped can be set
> > > > directly.
> > > 
> > > Sounds like the count number of extents request should be a separate
> > > ioctl and separate filesystem entry point instead of overloading FIEMAP.
> > > 
> > > Just define a simple FIECOUNT ioctl.
> > 
> > What about the back-end though? This is pretty transparently handled in
> > fiemap_fill_next_extent() and many file systems (Ocfs2 included) would just
> > have ->fiecount callbacks that are nearly identical ->fiecount to their
> > ->fiemap...
> 
> 	Provide generic_fiemap_fiecount() that does the operation in
> terms of ->fiemap().  Then filesystems like ocfs2 can just do .fiecount
> = generic_fiemap_fiecount.
> 	I agree with Christoph that it seems a bit overloaded when done
> as a special case of FIEMAP.

The question is whether there are any (or many) filesystems that will NOT
implement ->fiecount() as a wrapper of ->fiemap()?  At that point the
"simplification" of the API means that there is actually more code and
layers being called for the simple version.

I'm not saying I'm against having a FIECOUNT ioctl, we're probably just
going to use an updated filefrag like everyone else, but it doesn't seem
like a net reduction in code anywhere.

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