Occasionally improvments are made to how git-annex stores its data,
that require an upgrade process to convert repositories made with an older
version to be used by a newer version. It's annoying, it should happen
rarely, but sometimes, it's worth it.

There's a committment that git-annex will always support upgrades from all
past versions. After all, you may have offline drives from an earlier
git-annex, and might want to use them with a newer git-annex.

## Upgrade process

git-annex will automatically notice if it is run in a repository that
needs an upgrade, and perform the upgrade before running whatever it
was asked to do. Or you can use the "git annex upgrade" command to
explicitly do an upgrade. The upgrade can tend to take a while,
if you have a lot of files.

Each clone of a repository should be individually upgraded.
Until a repository's remotes have been upgraded, git-annex
may refuse to communicate with them.

Generally, start by upgrading one repository, and then you can commit
the changes git-annex staged during upgrade, and push them out to other
repositories. And then upgrade those other repositories. Doing it this
way avoids git-annex doing some duplicate work during the upgrade.

The upgrade process is guaranteed to be conflict-free. Unless you
already have git conflicts in your repository or between repositories.
Upgrading a repository with conflicts is not recommended; resolve the
conflicts first before upgrading git-annex.

Example upgrade process:

	cd localrepo
	git pull
	git annex upgrade
	(Upgrading object directory layout v1 to v2...)
	git commit -m "upgrade v1 to v2"
	git push

	ssh remote
	cd remoterepo
	git pull
	git annex upgrade
	...

## Upgrade events, so far

### v1 -> v2 (git-annex version 0.23 to version 0.20110316)

Involved adding hashing to .git/annex/ and changing the names of all keys.
Symlinks changed.

Also, hashing was added to location log files in .git-annex/.
And .gitattributes needed to have another line added to it.

Handled transparently.

### v0 -> v1 (git-annex version 0.03 to version 0.04)

Involved a reorganisation of the layout of .git/annex/. Symlinks changed.

Handled more or less transparently, although git-annex was just 2 weeks
old at the time, and had few users other than Joey.

This upgrade is believed to still be supported, but has not been tested
lately.