.. SPDX-License-Identifier: GPL-2.0
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================================
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Optimized MPEG Filesystem (OMFS)
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================================
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Overview
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========
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OMFS is a filesystem created by SonicBlue for use in the ReplayTV DVR
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and Rio Karma MP3 player. The filesystem is extent-based, utilizing
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block sizes from 2k to 8k, with hash-based directories. This
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filesystem driver may be used to read and write disks from these
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devices.
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Note, it is not recommended that this FS be used in place of a general
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filesystem for your own streaming media device. Native Linux filesystems
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will likely perform better.
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More information is available at:
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http://linux-karma.sf.net/
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Various utilities, including mkomfs and omfsck, are included with
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omfsprogs, available at:
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https://bobcopeland.com/karma/
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Instructions are included in its README.
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Options
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=======
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OMFS supports the following mount-time options:
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============ ========================================
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uid=n make all files owned by specified user
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gid=n make all files owned by specified group
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umask=xxx set permission umask to xxx
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fmask=xxx set umask to xxx for files
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dmask=xxx set umask to xxx for directories
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============ ========================================
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Disk format
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===========
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OMFS discriminates between "sysblocks" and normal data blocks. The sysblock
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group consists of super block information, file metadata, directory structures,
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and extents. Each sysblock has a header containing CRCs of the entire
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sysblock, and may be mirrored in successive blocks on the disk. A sysblock may
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have a smaller size than a data block, but since they are both addressed by the
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same 64-bit block number, any remaining space in the smaller sysblock is
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unused.
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Sysblock header information::
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struct omfs_header {
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__be64 h_self; /* FS block where this is located */
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__be32 h_body_size; /* size of useful data after header */
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__be16 h_crc; /* crc-ccitt of body_size bytes */
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char h_fill1[2];
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u8 h_version; /* version, always 1 */
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char h_type; /* OMFS_INODE_X */
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u8 h_magic; /* OMFS_IMAGIC */
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u8 h_check_xor; /* XOR of header bytes before this */
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__be32 h_fill2;
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};
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Files and directories are both represented by omfs_inode::
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struct omfs_inode {
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struct omfs_header i_head; /* header */
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__be64 i_parent; /* parent containing this inode */
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__be64 i_sibling; /* next inode in hash bucket */
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__be64 i_ctime; /* ctime, in milliseconds */
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char i_fill1[35];
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char i_type; /* OMFS_[DIR,FILE] */
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__be32 i_fill2;
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char i_fill3[64];
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char i_name[OMFS_NAMELEN]; /* filename */
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__be64 i_size; /* size of file, in bytes */
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};
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Directories in OMFS are implemented as a large hash table. Filenames are
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hashed then prepended into the bucket list beginning at OMFS_DIR_START.
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Lookup requires hashing the filename, then seeking across i_sibling pointers
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until a match is found on i_name. Empty buckets are represented by block
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pointers with all-1s (~0).
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A file is an omfs_inode structure followed by an extent table beginning at
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OMFS_EXTENT_START::
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struct omfs_extent_entry {
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__be64 e_cluster; /* start location of a set of blocks */
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__be64 e_blocks; /* number of blocks after e_cluster */
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};
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struct omfs_extent {
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__be64 e_next; /* next extent table location */
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__be32 e_extent_count; /* total # extents in this table */
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__be32 e_fill;
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struct omfs_extent_entry e_entry; /* start of extent entries */
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};
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Each extent holds the block offset followed by number of blocks allocated to
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the extent. The final extent in each table is a terminator with e_cluster
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being ~0 and e_blocks being ones'-complement of the total number of blocks
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in the table.
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If this table overflows, a continuation inode is written and pointed to by
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e_next. These have a header but lack the rest of the inode structure.
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