Re: newbie questions about git design and features (some wrt hg)

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Feb 1, 2007, 16:28 UTC
Message-ID: <Pine.LNX.4.64.0702010814470.3632@woody.linux-foundation.org>
In-Reply-To: <45C1BDD3.8050103@fs.ei.tum.de>

On Thu, 1 Feb 2007, Simon 'corecode' Schubert wrote:

Show 7 quoted lines

>
> > So, can you explain to me how a filename is _not_ a file-id?
> 
> It is not a file-id like other SCM use it (I think monotone, not sure though).
> If you copy/move the content to a new name, the ID will not stay the same.
> Just see it as a hash bucket which allows you easy access to the history for a
> file currently with this name.

Well, that's actually just another "file ID" too. It's just not an "inode number" kind of file ID, it's more the "CVS file ID" kind of ID.

SVN uses "inode numbers" (I think they are just UUID's generated at "svn add" time, but I'm not sure) to track file ID's across renames. Some other SCM's do the same.

CVS uses "pathname" as the file ID (which obviously doesn't need any separate generation at all), which is why you have to do horrible things to track file ID's across renames (ie you really can't, but you *can* copy or move the *,v file so that your *new* "file ID" also has the same history as your old one).

So both of those are "file ID's" - they are what is used to index into the history, and they have real meaning for very fundamental operations.

You can view git as "closer" to CVS, in the sense that it certainly doesn't have the SVN kind of location-independent ID, and it _is_ able to look back in history using the path-name. So in that sense, you can certainly claim that the pathname is the "file ID" in git too, and that git is closer to CVS than to SVN.

But unlike SVN or CVS, there is no real fundamental "meaning" to the pathname in git. Sure, you can use the pathname to trace history of a file, but on the other hand, you can use a random aggregation of pathnames to track history of a set of files and directories, and the pathnames actually exist even when the file doesn't. So there obviously isn't any 1:1 relationship, neither in usage, nor in any internal implementation.

So at least for me, "file ID" means "identifier for a particular chain of history". THAT exists in both CVS and SVN (it's a pathname and an "inode number" respectively), but does not exist in git at all.

			Linus