On 12/09/12 12:38, Hugo Mills wrote:

On Sun, Dec 09, 2012 at 12:20:46PM +0100, Swâmi Petaramesh wrote:

quoted

Le 09/12/2012 11:41, Roman Mamedov a écrit :

quoted

CoW filesystem incurs fragmentation by its nature, not specifically snapshots.
Even without snapshots, rewriting portions of existing files will write the
new blocks not over the original ones, but elsewhere, thus increasing
fragmentation.

Is it to expect that somewhere in the future, BTRFS will be able to
defragment itself without duplicating snapshot data ?

    In the presence of snapshots that are modified, no, it's impossible
to fully defrag all the files.

Of course, but I would agree with the poster that it would be important 
to "partially" defrag all the files, avoiding at least unneeded 
fragmentation...
At least the fragmentation generated by normal writes