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authorBob Copeland <me@bobcopeland.com>2008-07-25 22:45:14 -0400
committerLinus Torvalds <torvalds@linux-foundation.org>2008-07-26 15:00:05 -0400
commita14e4b572b0ee5c6dbe4aceb83d00b2c969324e9 (patch)
treee1745934d88e72a8e2fc1491b4c01c3fda5e2177 /Documentation/filesystems/omfs.txt
parent080ccd4573607a930367c2128fc709814b2ade5d (diff)
omfs: add filesystem documentation
These patches add the Optimized MPEG Filesystem, a proprietary filesystem used by the embedded devices Rio Karma and ReplayTV, which are no longer manufactured. This filesystem module enables people to access files on these devices. This patch: OMFS is a proprietary filesystem created for the ReplayTV and also used by the Rio Karma. It uses hash tables with unordered, unbounded lists in each bucket for directories, extents for data blocks, 64-bit addressing for blocks, with up to 8K blocks (only 2K of a given block is ever used for metadata, so the FS still works with 4K pages). Document the filesystem usage and structures. Signed-off-by: Bob Copeland <me@bobcopeland.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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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