projects / linux-block.git / blame
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| 7cbb468f MCC |
1 | .. SPDX-License-Identifier: GPL-2.0 |
| 2 | ||
| 3 | ================================ | |
| 4 | Optimized MPEG Filesystem (OMFS) | |
| 5 | ================================ | |
| 6 | ||
| 7 | Overview | |
| 8 | ======== | |
| 9 | ||
| 10 | OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR | |
| 11 | and Rio Karma MP3 player. The filesystem is extent-based, utilizing | |
| 12 | block sizes from 2k to 8k, with hash-based directories. This | |
| 13 | filesystem driver may be used to read and write disks from these | |
| 14 | devices. | |
| 15 | ||
| 16 | Note, it is not recommended that this FS be used in place of a general | |
| 17 | filesystem for your own streaming media device. Native Linux filesystems | |
| 18 | will likely perform better. | |
| 19 | ||
| 20 | More information is available at: | |
| 21 | ||
| 22 | http://linux-karma.sf.net/ | |
| 23 | ||
| 24 | Various utilities, including mkomfs and omfsck, are included with | |
| 25 | omfsprogs, available at: | |
| 26 | ||
| 561a75ac | 27 | https://bobcopeland.com/karma/ |
| 7cbb468f MCC |
28 | |
| 29 | Instructions are included in its README. | |
| 30 | ||
| 31 | Options | |
| 32 | ======= | |
| 33 | ||
| 34 | OMFS supports the following mount-time options: | |
| 35 | ||
| 36 | ============ ======================================== | |
| 37 | uid=n make all files owned by specified user | |
| 38 | gid=n make all files owned by specified group | |
| 39 | umask=xxx set permission umask to xxx | |
| 40 | fmask=xxx set umask to xxx for files | |
| 41 | dmask=xxx set umask to xxx for directories | |
| 42 | ============ ======================================== | |
| 43 | ||
| 44 | Disk format | |
| 45 | =========== | |
| 46 | ||
| 47 | OMFS discriminates between "sysblocks" and normal data blocks. The sysblock | |
| 48 | group consists of super block information, file metadata, directory structures, | |
| 49 | and extents. Each sysblock has a header containing CRCs of the entire | |
| 50 | sysblock, and may be mirrored in successive blocks on the disk. A sysblock may | |
| 51 | have a smaller size than a data block, but since they are both addressed by the | |
| 52 | same 64-bit block number, any remaining space in the smaller sysblock is | |
| 53 | unused. | |
| 54 | ||
| 55 | Sysblock header information:: | |
| 56 | ||
| 57 | struct omfs_header { | |
| 58 | __be64 h_self; /* FS block where this is located */ | |
| 59 | __be32 h_body_size; /* size of useful data after header */ | |
| 60 | __be16 h_crc; /* crc-ccitt of body_size bytes */ | |
| 61 | char h_fill1[2]; | |
| 62 | u8 h_version; /* version, always 1 */ | |
| 63 | char h_type; /* OMFS_INODE_X */ | |
| 64 | u8 h_magic; /* OMFS_IMAGIC */ | |
| 65 | u8 h_check_xor; /* XOR of header bytes before this */ | |
| 66 | __be32 h_fill2; | |
| 67 | }; | |
| 68 | ||
| 69 | Files and directories are both represented by omfs_inode:: | |
| 70 | ||
| 71 | struct omfs_inode { | |
| 72 | struct omfs_header i_head; /* header */ | |
| 73 | __be64 i_parent; /* parent containing this inode */ | |
| 74 | __be64 i_sibling; /* next inode in hash bucket */ | |
| 75 | __be64 i_ctime; /* ctime, in milliseconds */ | |
| 76 | char i_fill1[35]; | |
| 77 | char i_type; /* OMFS_[DIR,FILE] */ | |
| 78 | __be32 i_fill2; | |
| 79 | char i_fill3[64]; | |
| 80 | char i_name[OMFS_NAMELEN]; /* filename */ | |
| 81 | __be64 i_size; /* size of file, in bytes */ | |
| 82 | }; | |
| 83 | ||
| 84 | Directories in OMFS are implemented as a large hash table. Filenames are | |
| 85 | hashed then prepended into the bucket list beginning at OMFS_DIR_START. | |
| 86 | Lookup requires hashing the filename, then seeking across i_sibling pointers | |
| 87 | until a match is found on i_name. Empty buckets are represented by block | |
| 88 | pointers with all-1s (~0). | |
| 89 | ||
| 90 | A file is an omfs_inode structure followed by an extent table beginning at | |
| 91 | OMFS_EXTENT_START:: | |
| 92 | ||
| 93 | struct omfs_extent_entry { | |
| 94 | __be64 e_cluster; /* start location of a set of blocks */ | |
| 95 | __be64 e_blocks; /* number of blocks after e_cluster */ | |
| 96 | }; | |
| 97 | ||
| 98 | struct omfs_extent { | |
| 99 | __be64 e_next; /* next extent table location */ | |
| 100 | __be32 e_extent_count; /* total # extents in this table */ | |
| 101 | __be32 e_fill; | |
| 102 | struct omfs_extent_entry e_entry; /* start of extent entries */ | |
| 103 | }; | |
| 104 | ||
| 105 | Each extent holds the block offset followed by number of blocks allocated to | |
| 106 | the extent. The final extent in each table is a terminator with e_cluster | |
| 107 | being ~0 and e_blocks being ones'-complement of the total number of blocks | |
| 108 | in the table. | |
| 109 | ||
| 110 | If this table overflows, a continuation inode is written and pointed to by | |
| 111 | e_next. These have a header but lack the rest of the inode structure. | |
| 112 |