Bazil is a userspace file system. This means the files you see when using Bazil do not have to actually exist as files on a local file system. This lets you do things like browse terabytes of archived data on a laptop with a small SSD.
Your local disk will be used as the default storage location and a cache.
Each file stored in Bazil is divided into chunks, hashed in a Merkle tree, and stored into a Content-Addressed Store (CAS). Chunk sizes may vary across files, but each file uses just one chunk size; this is used for fast random access inside the file.
Using a CAS means that storing the content more than once does not take up any extra space (if using the same chunk size). Using a Merkle tree means that small updates to the file only need to store the updated chunks.
The Merkle tree also ensures data integrity. Knowing the hash at the root of the tree means none of the content below can have changed, maliciously or by bitrot.
Convergent encryption means data is encrypted with a secret key derived from the data itself. Bazil uses an extra secret to limit the scope to people you wish to share and/or deduplicate data with.
Each chunk stored in CAS is encrypted and authenticated with a NaCl secretbox, with a configured secret and a nonce derived from a personalized Blake2 hash of the key (which is another personalized Blake2 hash of the content). The encrypted data is identified by a new key, which is again a personalized Blake2 hash of the old key, keyed by the configured secret.
Trickling every single file content change as Merkle trees of directories all the way to the top of the root of the volume would be a lot of extra work. Instead, Bazil takes the hybrid approach: directories are stored in the CAS only when taking a snapshot; live data lives in a key-value database.
The directory contents are stored as <dir_inode><basename> ->
<file_metadata>. This makes readdir+stat fast, while always
serving directories in alphabetical order.
There is currently no support for multiple directory entries pointing to the same inode, aka hard links.
Taking a snapshot writes the directory contents from the database to the CAS, thereby archiving a snapshot of the state of the volume.
Snapshots are created and accessed through the top-level .snap
pseudo-directory. A new snapshot can be taken simply with mkdir.
$ echo Hello >greeting
$ mkdir .snap/remember-me
$ rm greeting
$ cat .snap/remember-me/greeting
Hello
The inode space is partitioned so that this and other dynamic content gets distinct inode numbers that never collide with allocated inodes.