1Optimized MPEG Filesystem (OMFS)
2
3Overview
4========
5
6OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR
7and Rio Karma MP3 player.  The filesystem is extent-based, utilizing
8block sizes from 2k to 8k, with hash-based directories.  This
9filesystem driver may be used to read and write disks from these
10devices.
11
12Note, it is not recommended that this FS be used in place of a general
13filesystem for your own streaming media device.  Native Linux filesystems
14will likely perform better.
15
16More information is available at:
17
18    http://linux-karma.sf.net/
19
20Various utilities, including mkomfs and omfsck, are included with
21omfsprogs, available at:
22
23    http://bobcopeland.com/karma/
24
25Instructions are included in its README.
26
27Options
28=======
29
30OMFS supports the following mount-time options:
31
32    uid=n        - make all files owned by specified user
33    gid=n        - make all files owned by specified group
34    umask=xxx    - set permission umask to xxx
35    fmask=xxx    - set umask to xxx for files
36    dmask=xxx    - set umask to xxx for directories
37
38Disk format
39===========
40
41OMFS discriminates between "sysblocks" and normal data blocks.  The sysblock
42group consists of super block information, file metadata, directory structures,
43and extents.  Each sysblock has a header containing CRCs of the entire
44sysblock, and may be mirrored in successive blocks on the disk.  A sysblock may
45have a smaller size than a data block, but since they are both addressed by the
46same 64-bit block number, any remaining space in the smaller sysblock is
47unused.
48
49Sysblock header information:
50
51struct omfs_header {
52        __be64 h_self;                  /* FS block where this is located */
53        __be32 h_body_size;             /* size of useful data after header */
54        __be16 h_crc;                   /* crc-ccitt of body_size bytes */
55        char h_fill1[2];
56        u8 h_version;                   /* version, always 1 */
57        char h_type;                    /* OMFS_INODE_X */
58        u8 h_magic;                     /* OMFS_IMAGIC */
59        u8 h_check_xor;                 /* XOR of header bytes before this */
60        __be32 h_fill2;
61};
62
63Files and directories are both represented by omfs_inode:
64
65struct omfs_inode {
66        struct omfs_header i_head;      /* header */
67        __be64 i_parent;                /* parent containing this inode */
68        __be64 i_sibling;               /* next inode in hash bucket */
69        __be64 i_ctime;                 /* ctime, in milliseconds */
70        char i_fill1[35];
71        char i_type;                    /* OMFS_[DIR,FILE] */
72        __be32 i_fill2;
73        char i_fill3[64];
74        char i_name[OMFS_NAMELEN];      /* filename */
75        __be64 i_size;                  /* size of file, in bytes */
76};
77
78Directories in OMFS are implemented as a large hash table.  Filenames are
79hashed then prepended into the bucket list beginning at OMFS_DIR_START.
80Lookup requires hashing the filename, then seeking across i_sibling pointers
81until a match is found on i_name.  Empty buckets are represented by block
82pointers with all-1s (~0).
83
84A file is an omfs_inode structure followed by an extent table beginning at
85OMFS_EXTENT_START:
86
87struct omfs_extent_entry {
88        __be64 e_cluster;               /* start location of a set of blocks */
89        __be64 e_blocks;                /* number of blocks after e_cluster */
90};
91
92struct omfs_extent {
93        __be64 e_next;                  /* next extent table location */
94        __be32 e_extent_count;          /* total # extents in this table */
95        __be32 e_fill;
96        struct omfs_extent_entry e_entry;       /* start of extent entries */
97};
98
99Each extent holds the block offset followed by number of blocks allocated to
100the extent.  The final extent in each table is a terminator with e_cluster
101being ~0 and e_blocks being ones'-complement of the total number of blocks
102in the table.
103
104If this table overflows, a continuation inode is written and pointed to by
105e_next.  These have a header but lack the rest of the inode structure.
106
107