This filesystem was designed for a few reasons. I am fed up with trying to implement an OpenFS driver, and my other filesystem designs (MiniFS and FAT-FS) have both been FAT-based. I want a proper, relatively full-featured, UNIX filesystem from which, perhaps, to run Monolith or some such thing.
Now, the specification.
All numbers are little-endian. All references to KB, MB, TB, GB, PB, EB, etc. refer to power-of-2 numbers because I am a programmer and I don’t like *iB.
OCFS uses inodes, sometimes known as index nodes, for file storage. All sectors excepting inodes and data sectors are unused. All references to “sector” are relative to a partition.
Sectors should be marked as used or unused - if the first byte of a sector is the ASCII character i, the sector is an inode. If the first byte is the ASCII character d, the sector contains file data. If the first byte is the ASCII character e, the sector contains extended file sector data. If the first byte is the ASCII null character \0, the sector is unused. These flags must be set when creating and deleting files.
Sector 1 holds some information about the filesystem. The root inode is stored on sector 2.
+------------------------------------------------+
| OCFS Superblock Format |
+--------+---------------------------------------+
| Bytes | Description |
+--------+---------------------------------------+
| 1-7 | Filesystem signature: ASCII string |
| | '\27OCFS\13\27' |
+--------+---------------------------------------+
| 8 | Filesystem flags - see 'Filesystem |
| | Flags' below. |
+--------+---------------------------------------+
| 9-512 | (8-byte chunks) Free sector cache. |
+--------+---------------------------------------+
+------------------------------------------------+
| Filesystem Flags |
+--------+---------------------------------------+
| Flag | Meaning |
+--------+---------------------------------------+
| 0x01 | Whether the filesystem is read-only. |
+--------+---------------------------------------+
| 0x02 | Whether the filesystem is encrypted. |
| | Encryption uses AES-256. |
+--------+---------------------------------------+
+------------------------------------------------+
| Inode Format |
+--------+---------------------------------------+
| Bytes | Description |
+--------+---------------------------------------+
| 1 | ASCII character 'i'; inode signature. |
+--------+---------------------------------------+
| 2 | File type. See 'File Types' below. |
+--------+---------------------------------------+
| 3-4 | File permissions, 2 bytes. See 'File |
| | Permissions' below. |
+--------+---------------------------------------+
| 5-8 | Owner UID, 4 bytes. |
+--------+---------------------------------------+
| 9-12 | Group ID, 4 bytes. |
+--------+---------------------------------------+
| 11-20 | POSIX 64-bit last modified timestamp. |
+--------+---------------------------------------+
| 21-64 | Case-sensitive filename, up to 44 |
| | ASCII characters or bytes. |
+--------+---------------------------------------+
| 65-504 | 8-byte chunks: If the inode describes |
| | a file, pointers to the file data |
| | sectors, in order. If the inode |
| | describes a directory, contains |
| | pointers to the directory's children, |
| | in order. Any excess should set to 0. |
+--------+---------------------------------------+
|505-512 | Pointer to a sector containing |
| | extensions of the previous pointer |
| | section; value should always be 0 if |
| | there is none. |
+--------+---------------------------------------+
+------------------------------------------------+
| File Types |
+--------+---------------------------------------+
| Byte | Meaning |
+--------+---------------------------------------+
| 0x00 | Invalid inode. |
+--------+---------------------------------------+
| 0x01 | Inode describes a text document, or |
| | file. |
+--------+---------------------------------------+
| 0x02 | Inode describes a directory. |
+--------+---------------------------------------+
| [TODO] | Add support for file linking |
+--------+---------------------------------------+
OCFS file permissions are laid out over 9 bits (the final 7 bits of the allocated two bytes are unused). If a bit is set, then the corresponding permission should be considered set.
+------------------------+
| BIT LAYOUT |
-------------------------+
| Bit | Permission |
+-----------+------------+
| 0x01 | Owner R |
+-----------+------------+
| 0x02 | Owner W |
+-----------+------------+
| 0x04 | Owner X |
+-----------+------------+
| 0x08 | Group R |
+-----------+------------+
| 0x10 | Group W |
+-----------+------------+
| 0x20 | Group X |
+-----------+------------+
| 0x40 | Other R |
+-----------+------------+
| 0x80 | Other W |
+-----------+------------+
| 0x100 | Other X |
+-----------+------------+