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.

git-annex will notice if it is run in a repository that
needs an upgrade, and refuse to do anything. To upgrade,
use the "git annex upgrade" command.

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.

## Upgrade events, so far

### v2 -> v3 (git-annex version 3.x)

Involved moving the .git-annex/ directory into a separate git-annex branch.

### tips for this upgrade

This upgrade is easier (and faster!) than the previous upgrades.
You don't need to upgrade every repository at once; it's sufficient
to upgrade each repository only when you next use it.

After the upgrade is complete, commit the changes it staged.
	
	git commit -m "upgrade v2 to v3"

Running `git gc` after this upgrade will likely free up significant disk 
space. (Tens to hundreds of megabytes.)

### v1 -> v2 (git-annex 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.

Previously, files added to the SHA [[backends]] did not have their file
size tracked, while files added to the WORM backend did. Files added to
the SHA backends after the conversion will have their file size tracked,
and that information will be used by git-annex for disk free space checking.
To ensure that information is available for all your annexed files, see
[[upgrades/SHA_size]].

### tips for this upgrade

This 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
will refuse to communicate with them.

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.

Example upgrade process:

	cd localrepo
	git pull
	git annex upgrade
	git commit -m "upgrade v1 to v2"
	git push

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

### v0 -> v1 (git-annex 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.