Git supports dividing the refs of a single repository into multiple namespaces, each of which has its own branches, tags, and HEAD. Git can expose each namespace as an independent repository to pull from and push to, while sharing the object store, and exposing all the refs to operations such as linkgit:git-gc[1].
Storing multiple repositories as namespaces of a single repository avoids storing duplicate copies of the same objects, such as when storing multiple branches of the same source. The alternates mechanism provides similar support for avoiding duplicates, but alternates do not prevent duplication between new objects added to the repositories without ongoing maintenance, while namespaces do.
To specify a namespace, set the GIT_NAMESPACE environment variable to
the namespace. For each ref namespace, Git stores the corresponding
refs in a directory under refs/namespaces/. For example,
GIT_NAMESPACE=foo will store refs under refs/namespaces/foo/. You
can also specify namespaces via the --namespace option to
linkgit:git[1].
Note that namespaces which include a / will expand to a hierarchy of
namespaces; for example, GIT_NAMESPACE=foo/bar will store refs under
refs/namespaces/foo/refs/namespaces/bar/. This makes paths in
GIT_NAMESPACE behave hierarchically, so that cloning with
GIT_NAMESPACE=foo/bar produces the same result as cloning with
GIT_NAMESPACE=foo and cloning from that repo with GIT_NAMESPACE=bar. It
also avoids ambiguity with strange namespace paths such as foo/refs/heads/,
which could otherwise generate directory/file conflicts within the refs
directory.
linkgit:git-upload-pack[1] and linkgit:git-receive-pack[1] rewrite the
names of refs as specified by GIT_NAMESPACE. git-upload-pack and
git-receive-pack will ignore all references outside the specified
namespace.
The smart HTTP server, linkgit:git-http-backend[1], will pass GIT_NAMESPACE through to the backend programs; see linkgit:git-http-backend[1] for sample configuration to expose repository namespaces as repositories.
For a simple local test, you can use linkgit:git-remote-ext[1]:
git clone ext::'git --namespace=foo %s /tmp/prefixed.git'