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.