1 This is the implementation of the SystemV/Coherent filesystem for Linux.
2 It grew out of separate filesystem implementations
3
4 Xenix FS Doug Evans <dje@cygnus.com> June 1992
5 SystemV FS Paul B. Monday <pmonday@eecs.wsu.edu> March-June 1993
6 Coherent FS B. Haible <haible@ma2s2.mathematik.uni-karlsruhe.de> June 1993
7
8 and was merged together in July 1993.
9
10 These filesystems are rather similar. Here is a comparison with Minix FS:
11
12 * Linux fdisk reports on partitions
13 - Minix FS 0x81 Linux/Minix
14 - Xenix FS ??
15 - SystemV FS ??
16 - Coherent FS 0x08 AIX bootable
17
18 * Size of a block or zone (data allocation unit on disk)
19 - Minix FS 1024
20 - Xenix FS 1024 (also 512 ??)
21 - SystemV FS 1024 (also 512 and 2048)
22 - Coherent FS 512
23
24 * General layout: all have one boot block, one super block and
25 separate areas for inodes and for directories/data.
26 On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
27 all the block numbers (including the super block) are offset by one track.
28
29 * Byte ordering of "short" (16 bit entities) on disk:
30 - Minix FS little endian 0 1
31 - Xenix FS little endian 0 1
32 - SystemV FS little endian 0 1
33 - Coherent FS little endian 0 1
34 Of course, this affects only the file system, not the data of files on it!
35
36 * Byte ordering of "long" (32 bit entities) on disk:
37 - Minix FS little endian 0 1 2 3
38 - Xenix FS little endian 0 1 2 3
39 - SystemV FS little endian 0 1 2 3
40 - Coherent FS PDP-11 2 3 0 1
41 Of course, this affects only the file system, not the data of files on it!
42
43 * Inode on disk: "short", 0 means non-existent, the root dir ino is:
44 - Minix FS 1
45 - Xenix FS, SystemV FS, Coherent FS 2
46
47 * Maximum number of hard links to a file:
48 - Minix FS 250
49 - Xenix FS ??
50 - SystemV FS ??
51 - Coherent FS >=10000
52
53 * Free inode management:
54 - Minix FS a bitmap
55 - Xenix FS, SystemV FS, Coherent FS
56 There is a cache of a certain number of free inodes in the super-block.
57 When it is exhausted, new free inodes are found using a linear search.
58
59 * Free block management:
60 - Minix FS a bitmap
61 - Xenix FS, SystemV FS, Coherent FS
62 Free blocks are organized in a "free list". Maybe a misleading term,
63 since it is not true that every free block contains a pointer to
64 the next free block. Rather, the free blocks are organized in chunks
65 of limited size, and every now and then a free block contains pointers
66 to the free blocks pertaining to the next chunk; the first of these
67 contains pointers and so on. The list terminates with a "block number"
68 0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
69
70 * Super-block location:
71 - Minix FS block 1 = bytes 1024..2047
72 - Xenix FS block 1 = bytes 1024..2047
73 - SystemV FS bytes 512..1023
74 - Coherent FS block 1 = bytes 512..1023
75
76 * Super-block layout:
77 - Minix FS
78 unsigned short s_ninodes;
79 unsigned short s_nzones;
80 unsigned short s_imap_blocks;
81 unsigned short s_zmap_blocks;
82 unsigned short s_firstdatazone;
83 unsigned short s_log_zone_size;
84 unsigned long s_max_size;
85 unsigned short s_magic;
86 - Xenix FS, SystemV FS, Coherent FS
87 unsigned short s_firstdatazone;
88 unsigned long s_nzones;
89 unsigned short s_fzone_count;
90 unsigned long s_fzones[NICFREE];
91 unsigned short s_finode_count;
92 unsigned short s_finodes[NICINOD];
93 char s_flock;
94 char s_ilock;
95 char s_modified;
96 char s_rdonly;
97 unsigned long s_time;
98 short s_dinfo[4]; -- SystemV FS only
99 unsigned long s_free_zones;
100 unsigned short s_free_inodes;
101 short s_dinfo[4]; -- Xenix FS only
102 unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
103 char s_fname[6];
104 char s_fpack[6];
105 then they differ considerably:
106 Xenix FS
107 char s_clean;
108 char s_fill[371];
109 long s_magic;
110 long s_type;
111 SystemV FS
112 long s_fill[12 or 14];
113 long s_state;
114 long s_magic;
115 long s_type;
116 Coherent FS
117 unsigned long s_unique;
118 Note that Coherent FS has no magic.
119
120 * Inode layout:
121 - Minix FS
122 unsigned short i_mode;
123 unsigned short i_uid;
124 unsigned long i_size;
125 unsigned long i_time;
126 unsigned char i_gid;
127 unsigned char i_nlinks;
128 unsigned short i_zone[7+1+1];
129 - Xenix FS, SystemV FS, Coherent FS
130 unsigned short i_mode;
131 unsigned short i_nlink;
132 unsigned short i_uid;
133 unsigned short i_gid;
134 unsigned long i_size;
135 unsigned char i_zone[3*(10+1+1+1)];
136 unsigned long i_atime;
137 unsigned long i_mtime;
138 unsigned long i_ctime;
139
140 * Regular file data blocks are organized as
141 - Minix FS
142 7 direct blocks
143 1 indirect block (pointers to blocks)
144 1 double-indirect block (pointer to pointers to blocks)
145 - Xenix FS, SystemV FS, Coherent FS
146 10 direct blocks
147 1 indirect block (pointers to blocks)
148 1 double-indirect block (pointer to pointers to blocks)
149 1 triple-indirect block (pointer to pointers to pointers to blocks)
150
151 * Inode size, inodes per block
152 - Minix FS 32 32
153 - Xenix FS 64 16
154 - SystemV FS 64 16
155 - Coherent FS 64 8
156
157 * Directory entry on disk
158 - Minix FS
159 unsigned short inode;
160 char name[14/30];
161 - Xenix FS, SystemV FS, Coherent FS
162 unsigned short inode;
163 char name[14];
164
165 * Dir entry size, dir entries per block
166 - Minix FS 16/32 64/32
167 - Xenix FS 16 64
168 - SystemV FS 16 64
169 - Coherent FS 16 32
170
171 * How to implement symbolic links such that the host fsck doesn't scream:
172 - Minix FS normal
173 - Xenix FS kludge: as regular files with chmod 1000
174 - SystemV FS ??
175 - Coherent FS kludge: as regular files with chmod 1000
176
177
178 Notation: We often speak of a "block" but mean a zone (the allocation unit)
179 and not the disk driver's notion of "block".
180
181
182 Bruno Haible <haible@ma2s2.mathematik.uni-karlsruhe.de>
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