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 mergefrom 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
annex.tune.branchhashdirectories. Most new tunables should take about 4
lines of code to add to git-annex.