phoenixfs

A filesystem implemented in userspace (using FUSE), inspired by the way Git is designed. This is a toy - expect it to be buggy.

Dependencies

1. Zlib (>= 1.2)
2. FUSE (>= 2.6)
3. pkg-config (>= 0.25)
4. Linux kernel (>= 2.6.15)

Usage

For the first run, you need two directories:

1. A git directory where the data will be stored.

2. An empty directory to use as the mountpoint.

   $ cd /tmp $ mkdir gitdir mountp $ phoneixfs mount gitdir mountp

Use the mountpoint as you see fit. Data will be written to the gitdir on umount: you can use it for subsequent mounts.

Now, everything in mountpoint is versioned. To access older revisions of FILE, use FILE@REV syntax, where REV is the number of revisions in the past you want to access. For example:

 $ echo "hello" >file1
 $ echo "goodbye" >file1
 $ echo "another hello" >file1
 $ cat file1
 another hello
 $ cat file1@1
 goodbye
 $ cat file1@2
 hello

Finally, to umount:

 $ fusermount -u mountp

Technical documentation

Uses a B+ tree to keep track of the filesystem tree, and a modified version of packfile v3/ packfile index v2 for storing revision information.

• gitdir/.git/loose/ contains zlib-deflated versions of content blobs, named by the SHA-1 digest of the content.

• gitdir/fstree is a raw dump of the B+ tree in a custom format.

• gitdir/master.pack and gitdir/master.idx are the packfile and packfile index respectively. During an unmount, the files in gitdir/.git/loose/ are packed up, and an index is generated.

• /tmp/phoenixfs.log is the debug log

master.idx, master.pack, and fstree are enough to recreate the entire versioned filesystem. The files in gitdir/ and gitdir/.git/loose/ can be removed after unmount.

Notes on the filesystem tree:

(dr: directory record | fr: file record | vfr: versioned file record)

• dr just contains a name and node pointer referencing vfrs. drs are inserted directly into the root node.

• vfr contains the path of the file, a list of frs representing the various versions of the file (fixed at REV_TRUNCATE), and a HEAD pointer to keep track of the latest version of the file.

• The B+ tree is keyed by the CRC32 hash of the path of the vfr/ dr, a design decision inspired by Btrfs.

• An fr, vfr, and dr (corresponding to its path) are created when a new file is created on the filesystem. Empty directories are not tracked: no dr is created for empty directories.

Limitations

The following invocations don't work:

 $ cp file1@1 file1 # FILE@REV can't be treated like a file

Contributing

Simply fork the project on GitHub and send pull requests.