This is the implementation of the SystemV/Coherent filesystem for Linux.
It grew out of separate filesystem implementations

    Xenix FS      Doug Evans <dje@cygnus.com>  June 1992
    SystemV FS    Paul B. Monday <pmonday@eecs.wsu.edu> March-June 1993
    Coherent FS   B. Haible <haible@ma2s2.mathematik.uni-karlsruhe.de> June 1993

and was merged together in July 1993.

These filesystems are rather similar. Here is a comparison with Minix FS:

* Linux fdisk reports on partitions
  - Minix FS     0x81 Linux/Minix
  - Xenix FS     ??
  - SystemV FS   ??
  - Coherent FS  0x08 AIX bootable

* Size of a block or zone (data allocation unit on disk)
  - Minix FS     1024
  - Xenix FS     1024 (also 512 ??)
  - SystemV FS   1024 (also 512)
  - Coherent FS   512

* General layout: all have one boot block, one super block and
  separate areas for inodes and for directories/data.
  On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
  all the block numbers (including the super block) are offset by one track.

* Byte ordering of "short" (16 bit entities) on disk:
  - Minix FS     little endian  0 1
  - Xenix FS     little endian  0 1
  - SystemV FS   little endian  0 1
  - Coherent FS  little endian  0 1
  Of course, this affects only the file system, not the data of files on it!

* Byte ordering of "long" (32 bit entities) on disk:
  - Minix FS     little endian  0 1 2 3
  - Xenix FS     little endian  0 1 2 3
  - SystemV FS   little endian  0 1 2 3
  - Coherent FS  PDP-11         2 3 0 1
  Of course, this affects only the file system, not the data of files on it!

* Inode on disk: "short", 0 means non-existent, the root dir ino is:
  - Minix FS                            1
  - Xenix FS, SystemV FS, Coherent FS   2

* Maximum number of hard links to a file:
  - Minix FS     250
  - Xenix FS     ??
  - SystemV FS   ??
  - Coherent FS  >=10000

* Free inode management:
  - Minix FS                             a bitmap
  - Xenix FS, SystemV FS, Coherent FS
      There is a cache of a certain number of free inodes in the super-block.
      When it is exhausted, new free inodes are found using a linear search.

* Free block management:
  - Minix FS                             a bitmap
  - Xenix FS, SystemV FS, Coherent FS
      Free blocks are organized in a "free list". Maybe a misleading term,
      since it is not true that every free block contains a pointer to
      the next free block. Rather, the free blocks are organized in chunks
      of limited size, and every now and then a free block contains pointers
      to the free blocks pertaining to the next chunk; the first of these
      contains pointers and so on. The list terminates with a "block number"
      0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.

* Super-block location:
  - Minix FS     block 1 = bytes 1024..2047
  - Xenix FS     block 1 = bytes 1024..2047
  - SystemV FS   bytes 512..1023
  - Coherent FS  block 1 = bytes 512..1023

* Super-block layout:
  - Minix FS
                    unsigned short s_ninodes;
                    unsigned short s_nzones;
                    unsigned short s_imap_blocks;
                    unsigned short s_zmap_blocks;
                    unsigned short s_firstdatazone;
                    unsigned short s_log_zone_size;
                    unsigned long s_max_size;
                    unsigned short s_magic;
  - Xenix FS, SystemV FS, Coherent FS
                    unsigned short s_firstdatazone;
                    unsigned long  s_nzones;
                    unsigned short s_fzone_count;
                    unsigned long  s_fzones[NICFREE];
                    unsigned short s_finode_count;
                    unsigned short s_finodes[NICINOD];
                    char           s_flock;
                    char           s_ilock;
                    char           s_modified;
                    char           s_rdonly;
                    unsigned long  s_time;
                    short          s_dinfo[4]; -- SystemV FS only
                    unsigned long  s_free_zones;
                    unsigned short s_free_inodes;
                    short          s_dinfo[4]; -- Xenix FS only
                    unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
                    char           s_fname[6];
                    char           s_fpack[6];
    then they differ considerably:
        Xenix FS
                    char           s_clean;
                    char           s_fill[371];
                    long           s_magic;
                    long           s_type;
        SystemV FS
                    long           s_fill[12 or 14];
                    long           s_state;
                    long           s_magic;
                    long           s_type;
        Coherent FS
                    unsigned long  s_unique;
    Note that Coherent FS has no magic.

* Inode layout:
  - Minix FS
                    unsigned short i_mode;
                    unsigned short i_uid;
                    unsigned long  i_size;
                    unsigned long  i_time;
                    unsigned char  i_gid;
                    unsigned char  i_nlinks;
                    unsigned short i_zone[7+1+1];
  - Xenix FS, SystemV FS, Coherent FS
                    unsigned short i_mode;
                    unsigned short i_nlink;
                    unsigned short i_uid;
                    unsigned short i_gid;
                    unsigned long  i_size;
                    unsigned char  i_zone[3*(10+1+1+1)];
                    unsigned long  i_atime;
                    unsigned long  i_mtime;
                    unsigned long  i_ctime;

* Regular file data blocks are organized as
  - Minix FS
               7 direct blocks
               1 indirect block (pointers to blocks)
               1 double-indirect block (pointer to pointers to blocks)
  - Xenix FS, SystemV FS, Coherent FS
              10 direct blocks
               1 indirect block (pointers to blocks)
               1 double-indirect block (pointer to pointers to blocks)
               1 triple-indirect block (pointer to pointers to pointers to blocks)

* Inode size, inodes per block
  - Minix FS        32   32
  - Xenix FS        64   16
  - SystemV FS      64   16
  - Coherent FS     64    8

* Directory entry on disk
  - Minix FS
                    unsigned short inode;
                    char name[14/30];
  - Xenix FS, SystemV FS, Coherent FS
                    unsigned short inode;
                    char name[14];

* Dir entry size, dir entries per block
  - Minix FS     16/32    64/32
  - Xenix FS     16       64
  - SystemV FS   16       64
  - Coherent FS  16       32

* How to implement symbolic links such that the host fsck doesn't scream:
  - Minix FS     normal
  - Xenix FS     kludge: as regular files with  chmod 1000
  - SystemV FS   ??
  - Coherent FS  kludge: as regular files with  chmod 1000


Notation: We often speak of a "block" but mean a zone (the allocation unit)
and not the disk driver's notion of "block".

Because the block size may be smaller than 1024 (which is the unit used by
the disk drivers and the buffer code), many functions must return a pointer
to the buffer data additionally to the buffer head pointer. One must not
assume that the entire buffer is occupied by this single block. This makes
the implementation of truncate() difficult.


Bruno Haible  <haible@ma2s2.mathematik.uni-karlsruhe.de>