0001 .. SPDX-License-Identifier: GPL-2.0
0002
0003 ==================
0004 SystemV Filesystem
0005 ==================
0006
0007 It implements all of
0008 - Xenix FS,
0009 - SystemV/386 FS,
0010 - Coherent FS.
0011
0012 To install:
0013
0014 * Answer the 'System V and Coherent filesystem support' question with 'y'
0015 when configuring the kernel.
0016 * To mount a disk or a partition, use::
0017
0018 mount [-r] -t sysv device mountpoint
0019
0020 The file system type names::
0021
0022 -t sysv
0023 -t xenix
0024 -t coherent
0025
0026 may be used interchangeably, but the last two will eventually disappear.
0027
0028 Bugs in the present implementation:
0029
0030 - Coherent FS:
0031
0032 - The "free list interleave" n:m is currently ignored.
0033 - Only file systems with no filesystem name and no pack name are recognized.
0034 (See Coherent "man mkfs" for a description of these features.)
0035
0036 - SystemV Release 2 FS:
0037
0038 The superblock is only searched in the blocks 9, 15, 18, which
0039 corresponds to the beginning of track 1 on floppy disks. No support
0040 for this FS on hard disk yet.
0041
0042
0043 These filesystems are rather similar. Here is a comparison with Minix FS:
0044
0045 * Linux fdisk reports on partitions
0046
0047 - Minix FS 0x81 Linux/Minix
0048 - Xenix FS ??
0049 - SystemV FS ??
0050 - Coherent FS 0x08 AIX bootable
0051
0052 * Size of a block or zone (data allocation unit on disk)
0053
0054 - Minix FS 1024
0055 - Xenix FS 1024 (also 512 ??)
0056 - SystemV FS 1024 (also 512 and 2048)
0057 - Coherent FS 512
0058
0059 * General layout: all have one boot block, one super block and
0060 separate areas for inodes and for directories/data.
0061 On SystemV Release 2 FS (e.g. Microport) the first track is reserved and
0062 all the block numbers (including the super block) are offset by one track.
0063
0064 * Byte ordering of "short" (16 bit entities) on disk:
0065
0066 - Minix FS little endian 0 1
0067 - Xenix FS little endian 0 1
0068 - SystemV FS little endian 0 1
0069 - Coherent FS little endian 0 1
0070
0071 Of course, this affects only the file system, not the data of files on it!
0072
0073 * Byte ordering of "long" (32 bit entities) on disk:
0074
0075 - Minix FS little endian 0 1 2 3
0076 - Xenix FS little endian 0 1 2 3
0077 - SystemV FS little endian 0 1 2 3
0078 - Coherent FS PDP-11 2 3 0 1
0079
0080 Of course, this affects only the file system, not the data of files on it!
0081
0082 * Inode on disk: "short", 0 means non-existent, the root dir ino is:
0083
0084 ================================= ==
0085 Minix FS 1
0086 Xenix FS, SystemV FS, Coherent FS 2
0087 ================================= ==
0088
0089 * Maximum number of hard links to a file:
0090
0091 =========== =========
0092 Minix FS 250
0093 Xenix FS ??
0094 SystemV FS ??
0095 Coherent FS >=10000
0096 =========== =========
0097
0098 * Free inode management:
0099
0100 - Minix FS
0101 a bitmap
0102 - Xenix FS, SystemV FS, Coherent FS
0103 There is a cache of a certain number of free inodes in the super-block.
0104 When it is exhausted, new free inodes are found using a linear search.
0105
0106 * Free block management:
0107
0108 - Minix FS
0109 a bitmap
0110 - Xenix FS, SystemV FS, Coherent FS
0111 Free blocks are organized in a "free list". Maybe a misleading term,
0112 since it is not true that every free block contains a pointer to
0113 the next free block. Rather, the free blocks are organized in chunks
0114 of limited size, and every now and then a free block contains pointers
0115 to the free blocks pertaining to the next chunk; the first of these
0116 contains pointers and so on. The list terminates with a "block number"
0117 0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.
0118
0119 * Super-block location:
0120
0121 =========== ==========================
0122 Minix FS block 1 = bytes 1024..2047
0123 Xenix FS block 1 = bytes 1024..2047
0124 SystemV FS bytes 512..1023
0125 Coherent FS block 1 = bytes 512..1023
0126 =========== ==========================
0127
0128 * Super-block layout:
0129
0130 - Minix FS::
0131
0132 unsigned short s_ninodes;
0133 unsigned short s_nzones;
0134 unsigned short s_imap_blocks;
0135 unsigned short s_zmap_blocks;
0136 unsigned short s_firstdatazone;
0137 unsigned short s_log_zone_size;
0138 unsigned long s_max_size;
0139 unsigned short s_magic;
0140
0141 - Xenix FS, SystemV FS, Coherent FS::
0142
0143 unsigned short s_firstdatazone;
0144 unsigned long s_nzones;
0145 unsigned short s_fzone_count;
0146 unsigned long s_fzones[NICFREE];
0147 unsigned short s_finode_count;
0148 unsigned short s_finodes[NICINOD];
0149 char s_flock;
0150 char s_ilock;
0151 char s_modified;
0152 char s_rdonly;
0153 unsigned long s_time;
0154 short s_dinfo[4]; -- SystemV FS only
0155 unsigned long s_free_zones;
0156 unsigned short s_free_inodes;
0157 short s_dinfo[4]; -- Xenix FS only
0158 unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only
0159 char s_fname[6];
0160 char s_fpack[6];
0161
0162 then they differ considerably:
0163
0164 Xenix FS::
0165
0166 char s_clean;
0167 char s_fill[371];
0168 long s_magic;
0169 long s_type;
0170
0171 SystemV FS::
0172
0173 long s_fill[12 or 14];
0174 long s_state;
0175 long s_magic;
0176 long s_type;
0177
0178 Coherent FS::
0179
0180 unsigned long s_unique;
0181
0182 Note that Coherent FS has no magic.
0183
0184 * Inode layout:
0185
0186 - Minix FS::
0187
0188 unsigned short i_mode;
0189 unsigned short i_uid;
0190 unsigned long i_size;
0191 unsigned long i_time;
0192 unsigned char i_gid;
0193 unsigned char i_nlinks;
0194 unsigned short i_zone[7+1+1];
0195
0196 - Xenix FS, SystemV FS, Coherent FS::
0197
0198 unsigned short i_mode;
0199 unsigned short i_nlink;
0200 unsigned short i_uid;
0201 unsigned short i_gid;
0202 unsigned long i_size;
0203 unsigned char i_zone[3*(10+1+1+1)];
0204 unsigned long i_atime;
0205 unsigned long i_mtime;
0206 unsigned long i_ctime;
0207
0208
0209 * Regular file data blocks are organized as
0210
0211 - Minix FS:
0212
0213 - 7 direct blocks
0214 - 1 indirect block (pointers to blocks)
0215 - 1 double-indirect block (pointer to pointers to blocks)
0216
0217 - Xenix FS, SystemV FS, Coherent FS:
0218
0219 - 10 direct blocks
0220 - 1 indirect block (pointers to blocks)
0221 - 1 double-indirect block (pointer to pointers to blocks)
0222 - 1 triple-indirect block (pointer to pointers to pointers to blocks)
0223
0224
0225 =========== ========== ================
0226 Inode size inodes per block
0227 =========== ========== ================
0228 Minix FS 32 32
0229 Xenix FS 64 16
0230 SystemV FS 64 16
0231 Coherent FS 64 8
0232 =========== ========== ================
0233
0234 * Directory entry on disk
0235
0236 - Minix FS::
0237
0238 unsigned short inode;
0239 char name[14/30];
0240
0241 - Xenix FS, SystemV FS, Coherent FS::
0242
0243 unsigned short inode;
0244 char name[14];
0245
0246 =========== ============== =====================
0247 Dir entry size dir entries per block
0248 =========== ============== =====================
0249 Minix FS 16/32 64/32
0250 Xenix FS 16 64
0251 SystemV FS 16 64
0252 Coherent FS 16 32
0253 =========== ============== =====================
0254
0255 * How to implement symbolic links such that the host fsck doesn't scream:
0256
0257 - Minix FS normal
0258 - Xenix FS kludge: as regular files with chmod 1000
0259 - SystemV FS ??
0260 - Coherent FS kludge: as regular files with chmod 1000
0261
0262
0263 Notation: We often speak of a "block" but mean a zone (the allocation unit)
0264 and not the disk driver's notion of "block".
