Starting from OSM snapshot timestamp 2012-08-30 0:00, delta files are distributed with zeroed new data. That means, that after decoding old gmapsupp.img file with the corresponding delta file, only parts of old file contained in the new file are copied. The rest is filled in with zeroes.
This strategy has two significant advantages:
- Delta files become much smaller and can be downloaded quickly. E.g. the whole set of delta files for the whole planet timestamp 2012-08-30 0:00 takes 456 Mb only while the previous set took more than 8 Gb.
- When old maps are patched, full maps torrent is used to download the rest of data. That moves majority of torrent downloads to the main cloud instead of downloading updates from pure diff clouds.
Vuze plugin can handle new diff files without problems. When appropriate delta file is downloaded by the plugin and the old map is patched, the whole set is rechecked and rest of data is downloaded from the maps torrent.
On the other hand, when you patch your maps yourself, copy the patched file to the appropriate place in the maps structure and recheck it according the maps torrent with the use of your BitTorrent client. Finally, download missing data.
Technical details concerning procedures used for zeroed delta files creation can be found here: https://code.google.com/p/xdeltaencoder/issues/detail?id=4