WSL2-Linux-Kernel/Documentation/filesystems/sysv-fs.rst

.. SPDX-License-Identifier: GPL-2.0

==================
SystemV Filesystem
==================

It implements all of
  - Xenix FS,
  - SystemV/386 FS,
  - Coherent FS.

To install:

* Answer the 'System V and Coherent filesystem support' question with 'y'
  when configuring the kernel.
* To mount a disk or a partition, use::

    mount [-r] -t sysv device mountpoint

  The file system type names::

               -t sysv
               -t xenix
               -t coherent

  may be used interchangeably, but the last two will eventually disappear.

Bugs in the present implementation:

- Coherent FS:

  - The "free list interleave" n:m is currently ignored.
  - Only file systems with no filesystem name and no pack name are recognized.
    (See Coherent "man mkfs" for a description of these features.)

- SystemV Release 2 FS:

  The superblock is only searched in the blocks 9, 15, 18, which
  corresponds to the beginning of track 1 on floppy disks. No support
  for this FS on hard disk yet.


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 and 2048)
  - 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".