Documentation/filesystems/sysv-fs.txt

   1 It implements all of
   2   - Xenix FS,
   3   - SystemV/386 FS,
   4   - Coherent FS.
   5
   6 To install:
   7 * Answer the 'System V and Coherent filesystem support' question with 'y'
   8   when configuring the kernel.
   9 * To mount a disk or a partition, use
  10     mount [-r] -t sysv device mountpoint
  11   The file system type names
  12                -t sysv
  13                -t xenix
  14                -t coherent
  15   may be used interchangeably, but the last two will eventually disappear.
  16
  17 Bugs in the present implementation:
  18 - Coherent FS:
  19   - The "free list interleave" n:m is currently ignored.
  20   - Only file systems with no filesystem name and no pack name are recognized.
  21   (See Coherent "man mkfs" for a description of these features.)
  22 - SystemV Release 2 FS:
  23   The superblock is only searched in the blocks 9, 15, 18, which
  24   corresponds to the beginning of track 1 on floppy disks. No support
  25   for this FS on hard disk yet.
  26
  27
  28 These filesystems are rather similar. Here is a comparison with Minix FS:
  29
  30 * Linux fdisk reports on partitions
  31   - Minix FS     0x81 Linux/Minix
  32   - Xenix FS     ??
  33   - SystemV FS   ??
  34   - Coherent FS  0x08 AIX bootable
  35
  36 * Size of a block or zone (data allocation unit on disk)
  37   - Minix FS     1024
  38   - Xenix FS     1024 (also 512 ??)
  39   - SystemV FS   1024 (also 512 and 2048)
  40   - Coherent FS   512
  41
  42 * General layout: all have one boot block, one super block and
  43   separate areas for inodes and for directories/data.
  44   On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
  45   all the block numbers (including the super block) are offset by one track.
  46
  47 * Byte ordering of "short" (16 bit entities) on disk:
  48   - Minix FS     little endian  0 1
  49   - Xenix FS     little endian  0 1
  50   - SystemV FS   little endian  0 1
  51   - Coherent FS  little endian  0 1
  52   Of course, this affects only the file system, not the data of files on it!
  53
  54 * Byte ordering of "long" (32 bit entities) on disk:
  55   - Minix FS     little endian  0 1 2 3
  56   - Xenix FS     little endian  0 1 2 3
  57   - SystemV FS   little endian  0 1 2 3
  58   - Coherent FS  PDP-11         2 3 0 1
  59   Of course, this affects only the file system, not the data of files on it!
  60
  61 * Inode on disk: "short", 0 means non-existent, the root dir ino is:
  62   - Minix FS                            1
  63   - Xenix FS, SystemV FS, Coherent FS   2
  64
  65 * Maximum number of hard links to a file:
  66   - Minix FS     250
  67   - Xenix FS     ??
  68   - SystemV FS   ??
  69   - Coherent FS  >=10000
  70
  71 * Free inode management:
  72   - Minix FS                             a bitmap
  73   - Xenix FS, SystemV FS, Coherent FS
  74       There is a cache of a certain number of free inodes in the super-block.
  75       When it is exhausted, new free inodes are found using a linear search.
  76
  77 * Free block management:
  78   - Minix FS                             a bitmap
  79   - Xenix FS, SystemV FS, Coherent FS
  80       Free blocks are organized in a "free list". Maybe a misleading term,
  81       since it is not true that every free block contains a pointer to
  82       the next free block. Rather, the free blocks are organized in chunks
  83       of limited size, and every now and then a free block contains pointers
  84       to the free blocks pertaining to the next chunk; the first of these
  85       contains pointers and so on. The list terminates with a "block number"
  86       0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
  87
  88 * Super-block location:
  89   - Minix FS     block 1 = bytes 1024..2047
  90   - Xenix FS     block 1 = bytes 1024..2047
  91   - SystemV FS   bytes 512..1023
  92   - Coherent FS  block 1 = bytes 512..1023
  93
  94 * Super-block layout:
  95   - Minix FS
  96                     unsigned short s_ninodes;
  97                     unsigned short s_nzones;
  98                     unsigned short s_imap_blocks;
  99                     unsigned short s_zmap_blocks;
 100                     unsigned short s_firstdatazone;
 101                     unsigned short s_log_zone_size;
 102                     unsigned long s_max_size;
 103                     unsigned short s_magic;
 104   - Xenix FS, SystemV FS, Coherent FS
 105                     unsigned short s_firstdatazone;
 106                     unsigned long  s_nzones;
 107                     unsigned short s_fzone_count;
 108                     unsigned long  s_fzones[NICFREE];
 109                     unsigned short s_finode_count;
 110                     unsigned short s_finodes[NICINOD];
 111                     char           s_flock;
 112                     char           s_ilock;
 113                     char           s_modified;
 114                     char           s_rdonly;
 115                     unsigned long  s_time;
 116                     short          s_dinfo[4]; -- SystemV FS only
 117                     unsigned long  s_free_zones;
 118                     unsigned short s_free_inodes;
 119                     short          s_dinfo[4]; -- Xenix FS only
 120                     unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
 121                     char           s_fname[6];
 122                     char           s_fpack[6];
 123     then they differ considerably:
 124         Xenix FS
 125                     char           s_clean;
 126                     char           s_fill[371];
 127                     long           s_magic;
 128                     long           s_type;
 129         SystemV FS
 130                     long           s_fill[12 or 14];
 131                     long           s_state;
 132                     long           s_magic;
 133                     long           s_type;
 134         Coherent FS
 135                     unsigned long  s_unique;
 136     Note that Coherent FS has no magic.
 137
 138 * Inode layout:
 139   - Minix FS
 140                     unsigned short i_mode;
 141                     unsigned short i_uid;
 142                     unsigned long  i_size;
 143                     unsigned long  i_time;
 144                     unsigned char  i_gid;
 145                     unsigned char  i_nlinks;
 146                     unsigned short i_zone[7+1+1];
 147   - Xenix FS, SystemV FS, Coherent FS
 148                     unsigned short i_mode;
 149                     unsigned short i_nlink;
 150                     unsigned short i_uid;
 151                     unsigned short i_gid;
 152                     unsigned long  i_size;
 153                     unsigned char  i_zone[3*(10+1+1+1)];
 154                     unsigned long  i_atime;
 155                     unsigned long  i_mtime;
 156                     unsigned long  i_ctime;
 157
 158 * Regular file data blocks are organized as
 159   - Minix FS
 160                7 direct blocks
 161                1 indirect block (pointers to blocks)
 162                1 double-indirect block (pointer to pointers to blocks)
 163   - Xenix FS, SystemV FS, Coherent FS
 164               10 direct blocks
 165                1 indirect block (pointers to blocks)
 166                1 double-indirect block (pointer to pointers to blocks)
 167                1 triple-indirect block (pointer to pointers to pointers to blocks)
 168
 169 * Inode size, inodes per block
 170   - Minix FS        32   32
 171   - Xenix FS        64   16
 172   - SystemV FS      64   16
 173   - Coherent FS     64    8
 174
 175 * Directory entry on disk
 176   - Minix FS
 177                     unsigned short inode;
 178                     char name[14/30];
 179   - Xenix FS, SystemV FS, Coherent FS
 180                     unsigned short inode;
 181                     char name[14];
 182
 183 * Dir entry size, dir entries per block
 184   - Minix FS     16/32    64/32
 185   - Xenix FS     16       64
 186   - SystemV FS   16       64
 187   - Coherent FS  16       32
 188
 189 * How to implement symbolic links such that the host fsck doesn't scream:
 190   - Minix FS     normal
 191   - Xenix FS     kludge: as regular files with  chmod 1000
 192   - SystemV FS   ??
 193   - Coherent FS  kludge: as regular files with  chmod 1000
 194
 195
 196 Notation: We often speak of a "block" but mean a zone (the allocation unit)
 197 and not the disk driver's notion of "block".