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)

  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];
    

  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".