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- Optimized MPEG Filesystem (OMFS)
- Overview
- ========
- 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:
- http://bobcopeland.com/karma/
- Instructions are included in its README.
- Options
- =======
- 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
- 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.
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