s3backer is a filesystem that contains a single file backed by the Amazon
Simple Storage Service (Amazon S3). As a filesystem, it is very simple:
it provides a single normal file having a fixed size. Underneath, the
file is divided up into blocks, and the content of each block is stored
in a unique Amazon S3 object. In other words, what s3backer provides is
really more like an S3-backed virtual hard disk device, rather than a
filesystem.

In typical usage, a `normal' filesystem is mounted on top of the file
exported by the s3backer filesystem using a loopback mount (or disk image
mount on Mac OS X).