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”.