Johannes Schindelin <Johannes.Schindelin@xxxxxx> writes: > On Tue, 14 Apr 2009, Patrick Berkeley wrote: > >> Does Git track the deltas on binary files? >> >> Someone in #git mentioned that if the binaries change too much Git no >> longer just stores the changes. If this is the case, what is the >> breaking point where Git goes from storing the deltas to the entire >> new file? > > Git does not store the deltas as you think it does. The deltification of > the objects is almost independent from the commmit history, i.e. we > _always_ store snapshots for most practical matters. Always store snapshots sounds as if you are not storing delta at all. I think I know what you meant to say, but the way you phrased it is misleading. Documentation/technical/pack-heuristics.txt talks about this in some detail. A short version is: - It does not make a difference if you are dealing with binary or text; - The delta is not necessarily against the same path in the previous revision, so even a new file added to the history can be stored in a delitified form; - When an object stored in the deltified representation is used, it would incur more cost than using the same object in the compressed base representation. The deltification mechanism makes a trade-off taking this cost into account, as well as the space efficiency. The last point may probably be not covered by pack-heuristics IRC talk Linus had in the documentation. Basically: - A deltified object is stored as an (compressed) xdelta against some base object. If the best deltified representation we come up with is larger than the result of just compressing the object without deltification, it is not worth storing it from the space comsumption point of view. Thus, we originally said something like "if an attempted delta is larger than half of the object size (assuming average 50% of compression ratio), do not use the deltified representation, it is not worth it". We attempt to delta against many base objects to pick the best possible delta; the number of attempt is called the delta window. - The base object of a deltified object could also be deltified, and you may need to repeatedly apply delta on top of some object that is not a delta to get to the final object. The length of this chain is called delta depth, and obviously you would want to keep the delta depth short to gain a reasonable runtime performance. Thus, when delitifying one object A, we make a weighted comparison between the size of the delta to build it out of an object of depth N and the size of the delta to build it out of an object of depth M. A slightly larger delta that is based on an object with a shallower delta depth is favored over a smaller delta based on an object with a much deeper delta depth. -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html