git-annex/doc/devblog/day_248__workload_tuning.mdwn

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 [[design/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.