Mercurial: Lightweight copies/renames

From Mercurial wiki - GSoC Ideas 2010:

Project Ideas

Lightweight copies/renames

(very dif开发者_Go百科ficult - a successful student will become an expert in Mercurial's storage format and transmission protocol)

Copies and renames currently are not too efficient. Mercurial copies the copied/renamed source file to the new initial revision of the target file in its internal history store. For renames, this is especially counter-intuitive, as renaming a large file grows the store by the file's size. It would be better if Mercurial had some way of referring to the existing revision from the new file, while preserving backwards compatbility and bounded I/O guarantees for retrieving revisions. See issue883 for discussion. There's an mq from an old attempt at this located here.

Sorry if this is an obvious question (I'm not good at English and programming). I'm wondering, what does the "Lightweight copies" mean?

Is it mean: when this feature is implemented, multiple files with same content (same hash value different file names) will be stored only once in repository (just like Git)?

Update:

Thanks everyone for your answers. One of Mercurial's developers - tonfa also answered this question in a comment of this answer:

caveman: When light-weight copies are implemented, will two files with same content (same hash value different names) store only once in repository (just like Git)?

tonfa: no, this feature isn't planned (it would break other optimizations to minimize disk access)

Right now, when you copy a file, a new file is created in the repository that contains a compressed snapshot of the file you just copied. The idea would be to set it up so the copy references the old file somehow and then has revlog entries based on that instead of having to have its own snapshot to base the revlog entries off of.

This will not be like how git works. Changing Mercurial to work that way would be really interesting, and not the easiest proposition.

I'd better say that copied/renamed file wouldn't store as twice space more as it now, but will just point to the same revision.

Not sure that this will be true for the files added separately with the same content. According to the description they will be treated as completely independent files and will occupy 2x space.