Re: [RFC][PATCH] Implement SEEK_HOLE/SEEK_DATA

[Nicholas Miell <nmiell@comcast.net>]

On Thu, 2007-11-29 at 00:48 +0100, Jörn Engel wrote:
> On Wed, 28 November 2007 16:39:59 -0700, Andreas Dilger wrote:
> > On Nov 28, 2007  14:56 -0800, Nicholas Miell wrote:
> > > 
> > > type: one of EXTENT_TYPE_HOLE, EXTENT_TYPE_DATA, EXTENT_TYPE_EXTENTS,
> > > EXTENT_TYPE_COMPRESSED, EXTENT_TYPE_UNCOMPRESSED etc.
> > 
> > This is what FIEMAP is supposed to do.  We wrote a spec and implemented
> > a prototype for ext4, but haven't had time to make it generic to move
> > the large part of the code into the VFS.  If someone wanted to take that
> > up, it would be much appreciated.
> > 
> > See "[RFC] add FIEMAP ioctl to efficiently map file allocation" in
> > linux-fsdevel for details on this interface.
> 
> I didn't follow the discussion much, since it didn't appear to suit
> logfs too well.  In a nutshell, logfs is purely block-based, prepends
> every block with a header, may compress blocks and packs them as tightly
> as possible (byte alignment).
> 
> Maybe the "MAP" part fooled me to believe FIEMAP would also expose
> physical location of extends on the medium.  But reading the proposal
> again, I am unsure about that part.  If physical locations are exposed,
> SEEK_HOLE/SEEK_DATA is significantly more elegant for logfs.  If not,
> FIEMAP could be useful.
> 
> Jörn
> 

I'd have to reread the original proposal, but I remember FIEMAP as being
a generalized way of getting information about a files extents. I think
the original proposal only dealt with mapping file offsets to physical
extents, but IIRC the interface was flexible enough to implement a
"where are the holes" request.

Regardless, SEEK_HOLE/SEEK_DATA being a better suited interface for the
needs of logfs doesn't make it the best interface for that need.

-- 
Nicholas Miell <nmiell@comcast.net>

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