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This is the implementation of the SystemV/Coherent filesystem for Linux.

It grew out of separate filesystem implementations

    Xenix FS      Doug Evans   June 1992

    SystemV FS    Paul B. Monday  March-June 1993

    Coherent FS   B. Haible  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"

* 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".

Bruno Haible