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Riak CS is S3 compatible.

I have a very similar problem and end up writing a python script that copies the files to cassandra and Nginx + Lua to serve them using lua-resty-cassandra. The read/write throughput is still not as good as I was expecting though.

You could try OpenStack's Swift project.

If you mean that you need something that supports the S3 API, you could try S3Proxy[0]. It basically presents the S3 API over a variety of different storage backends.

[0] https://github.com/andrewgaul/s3proxy