Documentation/filesystems/omfs.rst
.. SPDX-License-Identifier: GPL-2.0
OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR and Rio Karma MP3 player. The filesystem is extent-based, utilizing block sizes from 2k to 8k, with hash-based directories. This filesystem driver may be used to read and write disks from these devices.
Note, it is not recommended that this FS be used in place of a general filesystem for your own streaming media device. Native Linux filesystems will likely perform better.
More information is available at:
http://linux-karma.sf.net/
Various utilities, including mkomfs and omfsck, are included with omfsprogs, available at:
https://bobcopeland.com/karma/
Instructions are included in its README.
OMFS supports the following mount-time options:
============ ========================================
uid=n make all files owned by specified user
gid=n make all files owned by specified group
umask=xxx set permission umask to xxx
fmask=xxx set umask to xxx for files
dmask=xxx set umask to xxx for directories
============ ========================================
OMFS discriminates between "sysblocks" and normal data blocks. The sysblock group consists of super block information, file metadata, directory structures, and extents. Each sysblock has a header containing CRCs of the entire sysblock, and may be mirrored in successive blocks on the disk. A sysblock may have a smaller size than a data block, but since they are both addressed by the same 64-bit block number, any remaining space in the smaller sysblock is unused.
Sysblock header information::
struct omfs_header {
__be64 h_self; /* FS block where this is located */
__be32 h_body_size; /* size of useful data after header */
__be16 h_crc; /* crc-ccitt of body_size bytes */
char h_fill1[2];
u8 h_version; /* version, always 1 */
char h_type; /* OMFS_INODE_X */
u8 h_magic; /* OMFS_IMAGIC */
u8 h_check_xor; /* XOR of header bytes before this */
__be32 h_fill2;
};
Files and directories are both represented by omfs_inode::
struct omfs_inode {
struct omfs_header i_head; /* header */
__be64 i_parent; /* parent containing this inode */
__be64 i_sibling; /* next inode in hash bucket */
__be64 i_ctime; /* ctime, in milliseconds */
char i_fill1[35];
char i_type; /* OMFS_[DIR,FILE] */
__be32 i_fill2;
char i_fill3[64];
char i_name[OMFS_NAMELEN]; /* filename */
__be64 i_size; /* size of file, in bytes */
};
Directories in OMFS are implemented as a large hash table. Filenames are hashed then prepended into the bucket list beginning at OMFS_DIR_START. Lookup requires hashing the filename, then seeking across i_sibling pointers until a match is found on i_name. Empty buckets are represented by block pointers with all-1s (~0).
A file is an omfs_inode structure followed by an extent table beginning at OMFS_EXTENT_START::
struct omfs_extent_entry {
__be64 e_cluster; /* start location of a set of blocks */
__be64 e_blocks; /* number of blocks after e_cluster */
};
struct omfs_extent {
__be64 e_next; /* next extent table location */
__be32 e_extent_count; /* total # extents in this table */
__be32 e_fill;
struct omfs_extent_entry e_entry; /* start of extent entries */
};
Each extent holds the block offset followed by number of blocks allocated to the extent. The final extent in each table is a terminator with e_cluster being ~0 and e_blocks being ones'-complement of the total number of blocks in the table.
If this table overflows, a continuation inode is written and pointed to by e_next. These have a header but lack the rest of the inode structure.