Is there some script to find un-delta-able objects?

I.e. something to generate the .gitattributes file using this format:

https://git-scm.com/docs/gitattributes#_packing_objects

Some stuff is obvious, like "*.gpg binary -delta", but I'm wondering if there's some repo scanner utility to spew this out for a given repo.

On Fri, Oct 05, 2018 at 04:20:27PM +0200, Ævar Arnfjörð Bjarmason wrote:

I'm not sure what you mean by "un-delta-able" objects. Do you mean ones where we're not likely to find a delta? Or ones where Git will not try to look for a delta?

If the latter, I think the only rules are the "-delta" attribute and the object size. You should be able to use git-check-attr and "git-cat-file" to get that info.

If the former, I don't know how you would know. We can only report on what isn't a delta _yet_.

-Peff

On Fri, Oct 05 2018, Jeff King wrote:

Some version of the former. Ones where we haven't found any (or much of) useful deltas yet. E.g. say I had a repository with a lot of files generated by this command at various points in the history:

    dd if=/dev/urandom of=file.binary count=1024 bs=1024

Some script similar to git-sizer which could report that the packed+compressed+delta'd version of the 10 *.binary files I had in my history had a 1:1 ratio of how large they were in .git, v.s. how large the sum of each file retrieved by "git show" was (i.e. uncompressed, un-delta'd).

That doesn't mean that tomorrow I won't commit 10 new objects which would have a really good delta ratio to those 10 existing files, bringing the ratio to ~1:2, but if I had some report like:

    <ratio> <extension>

For a given repo that could be fed into .gitattributes to say we shouldn't bother to delta files of certain extensions.

On Fri, Oct 05, 2018 at 06:44:25PM +0200, Ævar Arnfjörð Bjarmason wrote:

You can get the uncompressed and on-disk sizes with:

  git cat-file --batch-all-objects \
    --batch-check='%(objectname) %(objectsize) %(objectsize:disk)'

and then compare the sizes/ratios however you like. If you want just a subset of the blobs, drop the "--batch-all-objects" and just feed the object names or even "HEAD:filename" on stdin).

I don't know of a tool that does that, but I think a modest application of perl to the cat-file output would produce it.

-Peff

Jeff King <peff@peff.net> writes:

I am reasonably sure that the question is about solving the former so that "-delta" attribute is set appropriately.

Iniitially, I thought that it is likely an undeltifiable object has higher randomness than deltifiable ones and that can be exploited, but if you have such a highly random blob A (and no other object like it) in the repository and then later acquire another blob B that happens to share most of the data with A, then A and B by themselves will pass the "highly random" test but still yet each can be expressed as a delta derived from the other. So your "what isn't a delta yet" is a reasonable assessment of what mechanically can be known.

Knowledge/heuristic like "No two '*.gpg' files are expected to be alike" needs something more than the randomness of individual files, I guess.