linux/Documentation/filesystems/sysv-fs.txt
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   1It implements all of
   2  - Xenix FS,
   3  - SystemV/386 FS,
   4  - Coherent FS.
   5
   6To 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
  17Bugs 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
  28These 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
 196Notation: We often speak of a "block" but mean a zone (the allocation unit)
 197and not the disk driver's notion of "block".
 198
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