path: root/Documentation/filesystems/omfs.txt
diff options
Diffstat (limited to 'Documentation/filesystems/omfs.txt')
1 files changed, 106 insertions, 0 deletions
diff --git a/Documentation/filesystems/omfs.txt b/Documentation/filesystems/omfs.txt
new file mode 100644
index 00000000..1d0d41ff
--- /dev/null
+++ b/Documentation/filesystems/omfs.txt
@@ -0,0 +1,106 @@
+Optimized MPEG Filesystem (OMFS)
+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
+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:
+ http://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
+Disk format
+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
+same 64-bit block number, any remaining space in the smaller sysblock is
+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
+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.