Is it possible to have a Subversion repository as a Git submodule?

No. Your best bet would be to set up a mirror of the svn repository in a dedicated git repository.

git svn clone -s http://subversion.example.com/ mysvnclone
cd mysvnclone
git remote add origin git@example.com:project.git
git push origin master

Then you can add the git repository as a submodule to the original project

cd /path/to/gitproject
git submodule add git://example.com/project.git -- svn-project
git add svn-project
git commit -m "Add submodule"

There is one conceptual difference between svn:externals and git submodule that may trip you up if you approach this from a subversion point of view. The git submodule is pegged to the revision that you give it. If "upstream" changes, then you have to update your submodule's reference.

So when we resync with the upstream subversion:

cd /path/to/mysvnclone
git svn rebase
git push

... the git project will still use the original revision that we committed earlier. To update to the svn HEAD, you would have to use

cd /path/to/gitproject/svn-project
git checkout master
git pull
cd ..
git add svn-project
git commit -m"Update submodule"

I just went through this. I'm doing something similar to rq, but slightly different. I setup one of my servers to host these git clones of the svn repos I need. In my case I only want read-only versions, and need a bare repo on the server.

On the server I run:

GIT_DIR=<projectname>.git git init
cd <projectname>.git/
GIT_DIR=. git svn init svn://example.com/trunk
GIT_DIR=. git svn fetch
git gc

This sets up my bare repo, then I have a cron script to update it:

#!/usr/bin/python

import os, glob

GIT_HOME='/var/www/git'

os.chdir(GIT_HOME)
os.environ['GIT_DIR']='.'
gits = glob.glob('*.git')
for git in gits:
  if not os.path.isdir(git):
    continue
  os.chdir(os.path.join(GIT_HOME, git))
  if not os.path.isdir('svn/git-svn'):
    #Not a git-svn repo
    continue

  #Pull in svn updates
  os.system('git svn fetch && git gc --quiet')
  #fix-svn-refs.sh makes all the svn branches/tags pullable
  os.system('fix-svn-refs.sh')
  #Update the master branch
  os.system('git fetch . +svn/git-svn:master && git gc --quiet')`

This also requires fix-svn-refs.sh from http://www.shatow.net/fix-svn-refs.sh This was mostly inspired by: http://gsocblog.jsharpe.net/archives/12

I'm not sure why the git gc is needed here, but I wasn't able to do a git pull without it.

So after all this you can then use git submodule following rq's instructions.

Piston is being rewritten to support this, and the converse, plus the existing Subversion URL in a Subvresion repoistory and git+git.

Check out the piston Github repository.

Unfortunately it doesn't seem to have been released.

Well, there is git-remote-testsvn, so I guess something like

git submodule add testsvn::http://www.telegraphics.com.au/svn/bzquips/trunk/ \
    module/bzquips

should work. Does it?