Today I put together a lot of things I've been thinking about:

• There's some evidence that git-annex needs tuning to handle some unusual repositories. In particular very big repositories might benefit from different object hashing.
• It's really hard to handle upgrades that change the fundamentals of how git-annex repositories work. Such an upgrade would need every git-annex user to upgrade their repository, and would be very painful. It's hard to imagine a change that is worth that amount of pain.
• There are other changes some would like to see (like lower-case object hash directory names) that are certainly not enough to warrant a flag day repo format upgrade.
• It would be nice to let people who want to have some flexibility to play around with changes, in their own repos, as long as they don't a) make git-annex a lot more complicated, or b) negatively impact others. (Without having to fork git-annex.)

This is discussed in more depth in new repo versions.

The solution, which I've built today, is support for tuning settings, when a new repository is first created. The resulting repository will be different in some significant way from a default git-annex repository, but git-annex will support it just fine.

The main limitations are:

• You can't change the tuning of an existing repository (unless a tool gets written to transition it).
• You absolutely don't want to merge repo B, which has been tuned in nonstandard ways, into repo A which has not. Or A into B. (Unless you like watching slow motion car crashes.)

I built all the infrastructure for this today. Basically, the git-annex branch gets a record of all tunings that have been applied, and they're automatically propagated to new clones of a repository.

And I implemented the first tunable setting:

git -c annex.tune.objecthashlower=true annex init

This is definitely an experimental feature for now. git-annex merge and similar commands will detect attempts to merge between incompatibly tuned repositories, and error out. But, there are a lot of ways to shoot yourself in the foot if you use this feature:

• Nothing stops git merge from merging two incompatible repositories.
• Nothing stops any version of git-annex older from today from merging either.

Now that the groundwork is laid, I can pretty easily, and inexpensively, add more tunable settings. The next two I plan to add are already documented, annex.tune.objecthashdirectories and annex.tune.branchhashdirectories. Most new tunables should take about 4 lines of code to add to git-annex.