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