Daemonic Dispatches Musings from Colin Percival Regular readers of these Daemonic Dispatches will no doubt have noticed that I have mentioned Amazon Web Services on many occasions , and it's no secret that my tarsnap online backup service is built on top of Amazon Web Services. Over the month since tarsnap reached public beta , a number of people have asked me questions about AWS and how tarsnap uses it, so I think now is a good time to provide some insight into how the tarsnap service works behind the scenes. The tarsnap server provides a transactional key -> blob store to tarsnap clients. The keys are a fixed 33 bytes (a one-character type plus a 256-bit unique ID generated using SHA256), while blobs are an average of about 30 kB but can be as large as 256 kB. In order to create a new archive, the tarsnap client sends a "write transaction start" request, many "write data...