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Based on kernel version 3.9. Page generated on 2013-05-02 23:06 EST.

1	Optimized MPEG Filesystem (OMFS)
2	
3	Overview
4	========
5	
6	OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR
7	and Rio Karma MP3 player.  The filesystem is extent-based, utilizing
8	block sizes from 2k to 8k, with hash-based directories.  This
9	filesystem driver may be used to read and write disks from these
10	devices.
11	
12	Note, it is not recommended that this FS be used in place of a general
13	filesystem for your own streaming media device.  Native Linux filesystems
14	will likely perform better.
15	
16	More information is available at:
17	
18	    http://linux-karma.sf.net/
19	
20	Various utilities, including mkomfs and omfsck, are included with
21	omfsprogs, available at:
22	
23	    http://bobcopeland.com/karma/
24	
25	Instructions are included in its README.
26	
27	Options
28	=======
29	
30	OMFS 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	
38	Disk format
39	===========
40	
41	OMFS discriminates between "sysblocks" and normal data blocks.  The sysblock
42	group consists of super block information, file metadata, directory structures,
43	and extents.  Each sysblock has a header containing CRCs of the entire
44	sysblock, and may be mirrored in successive blocks on the disk.  A sysblock may
45	have a smaller size than a data block, but since they are both addressed by the
46	same 64-bit block number, any remaining space in the smaller sysblock is
47	unused.
48	
49	Sysblock header information:
50	
51	struct 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	
63	Files and directories are both represented by omfs_inode:
64	
65	struct 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	
78	Directories in OMFS are implemented as a large hash table.  Filenames are
79	hashed then prepended into the bucket list beginning at OMFS_DIR_START.
80	Lookup requires hashing the filename, then seeking across i_sibling pointers
81	until a match is found on i_name.  Empty buckets are represented by block
82	pointers with all-1s (~0).
83	
84	A file is an omfs_inode structure followed by an extent table beginning at
85	OMFS_EXTENT_START:
86	
87	struct 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	
92	struct 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	
99	Each extent holds the block offset followed by number of blocks allocated to
100	the extent.  The final extent in each table is a terminator with e_cluster
101	being ~0 and e_blocks being ones'-complement of the total number of blocks
102	in the table.
103	
104	If this table overflows, a continuation inode is written and pointed to by
105	e_next.  These have a header but lack the rest of the inode structure.
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