This library and binary create FactoryFS partitions by exporting existing directory structures. FactoryFS is immutable, so using this tool is the only real way to create these partitions with data in them.
The library has a single call, export_directory, which takes a DirectoryProxy, which is the root of the directory tree you want to export, and a channel to a block device.
The binary can be included in the build by adding --with //src/storage:tools to your fx set line. It takes two arguments - one, a path to a directory to export, two, a path to a block device to write the data to. This binary is intended to be used for manual development and debugging of the factoryfs library.
$ fx set core.x64 --with //src/storage:tools $ fx build # on fuchsia $ export-ffs /path/to/export /dev/class/block/<id>