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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?

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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.

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  • I thought journaling file systems kept meticulous logs. Either way, a file system driver should work right?
    – Milind R
    Nov 3, 2015 at 11:56
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    @MilindR they do, but not forever - where would they keep them? You'd fill up your disk with journals. So they keep data in the transaction log file only until the data is flushed to disk, then the log is reset. See this page about NTFS journalling
    – gbjbaanb
    Nov 3, 2015 at 12:01
  • Useful link, thanks. The very robustness seen there is what prompted me to wonder about piggybacking on it. So we can capture the operations happening in a local repository till the next commit, and then flush?
    – Milind R
    Nov 4, 2015 at 4:20
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    You'd have to write such a thing as part of the filesystem driver - I doubt you'd have enough control over when the log was flushed to reliably use it for userland purposes.
    – gbjbaanb
    Nov 4, 2015 at 8:35
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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.

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  • True. Two ways of mitigating that : 1) use a file system driver to capture how much ever is possible. 2) use both approaches, and form a master list with the more fine grained of the two.
    – Milind R
    Nov 3, 2015 at 11:55
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    How does having a long and fine-grained list of changes help you solve the merging problem? It only makes it worse.
    – Aleksandr Dubinsky
    Nov 3, 2015 at 11:58
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    @MilindR As an additional indicator, it may be helpful, but like I hinted at, there's multiple ways to "rename" a file, and simply looking at the fs operations will only reveal the most obvious way. But you didn't answer my question. Beside tracking renames/moves, what else will you accomplish? Those are easy to detect anyway (edit distance). The tough part is handling merge conflicts. What might be most useful there is not going lower-level, but going higher. Ie, tracking refactoring operations like renaming of variables.
    – Aleksandr Dubinsky
    Nov 3, 2015 at 15:47
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    @MilindR I would disagree with you there. No matter what I did with the file, if the content of the new file is the same, I would want my vcs history to be "optimized" to show a rename. I always aim for the history to be clean, and the minutiae of how I messed up my file manipulations especially should be hidden. God forbid I start to pay attention to that.
    – Aleksandr Dubinsky
    Nov 4, 2015 at 12:45
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    @MilindR I still doubt the file system history will reveal this intent. Why wouldn't I just rename the file with the intent of giving it a different purpose? It would be the easiest way to get rid of the old file and create a new one with the same (or similar) content. If you'd like the rename status to communicate intent, it would make sense to let the user specify/modify that while making the commit.
    – Aleksandr Dubinsky
    Nov 5, 2015 at 21:31

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