I'm considering using a Docker image as way to snapshot a MySQL database after fixtures have been loaded, in order to quickly restore the snapshot before each acceptance test.
In order for this to be fast enough, the layer filesystem should operate at block level, not file level: if every (potentially large enough) database file has to be copied before being written to, this would be quite a bottleneck.
Do Docker layers operate at file level, or block level? Does this depend on the storage driver?