[PATCH] adding two new options to 'cp'

[Rick C. Petty]

On Wed, Aug 02, 2006 at 05:33:40PM +1000, Peter Jeremy wrote:
> On Tue, 2006-Aug-01 23:35:03 -0700, Tim Kientzle wrote:
> >The "cheap" solution is to handle it purely on
> >extract:  Detect blocks of zeros when restoring
> >files and seek over them.
> 
> The downside is that you wind up with a sparse file whether or not you
> wanted one.

No, you wind up with a sparse file if you specify the "-S" option to tar.
If you didn't want one, don't specify the option.

> Actually, the only real solution to copying sparse files is to add
> a system call that can return a map of holes.  This would neatly
> address the "needs two passes" problem with tar.

You don't need two passes-- you advance the file pointer whenever you
detect a block of zeros.  Note: care must be taken to only do this for
newly-created or otherwise truncated files, otherwise the skip ahead
wouldn't work.

> As a general comment (not addressed to Tim):  There _is_ a downside
> to sparsifying files.  If you take a sparse file and start filling
> in the holes, the net result will be very badly fragmented and hence
> have very poor sequential I/O performance.  If you're never going to
> update a file then making it sparse makes sense, if you will be
> updating it, you will get better performance by making it non-sparse.

Agreed, with a minor correction/elaboration.  By "updating" you mean
specifically "updating but not appending".  And another note:  a good way
to defragment a file is to sequentially copy it.  (The "best" way is to
copy the file to a new filesystem, that way you guarantee the blocks
allocated to the file are contiguous.)

-- Rick C. Petty