Can version control systems use the filesystem log to capture changes?

Question

I was trying to find a "perfect" syncing program between a network share and a local folder, when I realised that it's probably impossible to do it right unless all the filesystem operations were captured by a log of some sort, that could then be replayed on the remote server. Of course the same thing is required at the other end for retrieving changes from the remote server, but one could also use the existing system for that purpose, and if worst comes to worst, manual merging.

The advantage of this approach seems to be that handling deletes, renames, moves and copies, etc., become trivial, because the filesystem records these operations. Each commit would then consist of merely choosing the next meaningful changed version. The version control log would be a low-resolution version of the filesystem log, but will be more portable and persistent because it can be transported.

Is this viable, or have I understood the concepts and limitations wrong?

Answer 1

You've misunderstood because the filesystem transaction log is a very temporary beast, it does not hang around long enough to be used for history (at least in no filesystem I know of).

But otherwise, yes - its a valid principle. The VCS log would be the only log you have however. It cannot be transported though as it is not a transaction log, it a 'living' history of operations. You could transport the view of this history however.

Sync is already implemented like this - SVNSync does exactly the replay operations you describe.

Answer 2

File system operations are actually often at a coarser granularity than the changes that VCS tracks. A file may be over-written in a single operation, when only a line has really been changed.

Other times, the file system operations are too fine-grained and miss the big picture. Someone may "copy" a file and then rewrite everything except the copyright notice. Or they may delete a file, copy it from somewhere else under a new name but with the same contents.

VCS does the right thing in computing diffs directly on what's in the files rather than analyzing file system operations. The latter would be far more fragile and headache-inducing.