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1.. SPDX-License-Identifier: GPL-2.02 3==================4SystemV Filesystem5==================6 7It implements all of8 - Xenix FS,9 - SystemV/386 FS,10 - Coherent FS.11 12To install:13 14* Answer the 'System V and Coherent filesystem support' question with 'y'15 when configuring the kernel.16* To mount a disk or a partition, use::17 18 mount [-r] -t sysv device mountpoint19 20 The file system type names::21 22 -t sysv23 -t xenix24 -t coherent25 26 may be used interchangeably, but the last two will eventually disappear.27 28Bugs in the present implementation:29 30- Coherent FS:31 32 - The "free list interleave" n:m is currently ignored.33 - Only file systems with no filesystem name and no pack name are recognized.34 (See Coherent "man mkfs" for a description of these features.)35 36- SystemV Release 2 FS:37 38 The superblock is only searched in the blocks 9, 15, 18, which39 corresponds to the beginning of track 1 on floppy disks. No support40 for this FS on hard disk yet.41 42 43These filesystems are rather similar. Here is a comparison with Minix FS:44 45* Linux fdisk reports on partitions46 47 - Minix FS 0x81 Linux/Minix48 - Xenix FS ??49 - SystemV FS ??50 - Coherent FS 0x08 AIX bootable51 52* Size of a block or zone (data allocation unit on disk)53 54 - Minix FS 102455 - Xenix FS 1024 (also 512 ??)56 - SystemV FS 1024 (also 512 and 2048)57 - Coherent FS 51258 59* General layout: all have one boot block, one super block and60 separate areas for inodes and for directories/data.61 On SystemV Release 2 FS (e.g. Microport) the first track is reserved and62 all the block numbers (including the super block) are offset by one track.63 64* Byte ordering of "short" (16 bit entities) on disk:65 66 - Minix FS little endian 0 167 - Xenix FS little endian 0 168 - SystemV FS little endian 0 169 - Coherent FS little endian 0 170 71 Of course, this affects only the file system, not the data of files on it!72 73* Byte ordering of "long" (32 bit entities) on disk:74 75 - Minix FS little endian 0 1 2 376 - Xenix FS little endian 0 1 2 377 - SystemV FS little endian 0 1 2 378 - Coherent FS PDP-11 2 3 0 179 80 Of course, this affects only the file system, not the data of files on it!81 82* Inode on disk: "short", 0 means non-existent, the root dir ino is:83 84 ================================= ==85 Minix FS 186 Xenix FS, SystemV FS, Coherent FS 287 ================================= ==88 89* Maximum number of hard links to a file:90 91 =========== =========92 Minix FS 25093 Xenix FS ??94 SystemV FS ??95 Coherent FS >=1000096 =========== =========97 98* Free inode management:99 100 - Minix FS101 a bitmap102 - Xenix FS, SystemV FS, Coherent FS103 There is a cache of a certain number of free inodes in the super-block.104 When it is exhausted, new free inodes are found using a linear search.105 106* Free block management:107 108 - Minix FS109 a bitmap110 - Xenix FS, SystemV FS, Coherent FS111 Free blocks are organized in a "free list". Maybe a misleading term,112 since it is not true that every free block contains a pointer to113 the next free block. Rather, the free blocks are organized in chunks114 of limited size, and every now and then a free block contains pointers115 to the free blocks pertaining to the next chunk; the first of these116 contains pointers and so on. The list terminates with a "block number"117 0 on Xenix FS and SystemV FS, with a block zeroed out on Coherent FS.118 119* Super-block location:120 121 =========== ==========================122 Minix FS block 1 = bytes 1024..2047123 Xenix FS block 1 = bytes 1024..2047124 SystemV FS bytes 512..1023125 Coherent FS block 1 = bytes 512..1023126 =========== ==========================127 128* Super-block layout:129 130 - Minix FS::131 132 unsigned short s_ninodes;133 unsigned short s_nzones;134 unsigned short s_imap_blocks;135 unsigned short s_zmap_blocks;136 unsigned short s_firstdatazone;137 unsigned short s_log_zone_size;138 unsigned long s_max_size;139 unsigned short s_magic;140 141 - Xenix FS, SystemV FS, Coherent FS::142 143 unsigned short s_firstdatazone;144 unsigned long s_nzones;145 unsigned short s_fzone_count;146 unsigned long s_fzones[NICFREE];147 unsigned short s_finode_count;148 unsigned short s_finodes[NICINOD];149 char s_flock;150 char s_ilock;151 char s_modified;152 char s_rdonly;153 unsigned long s_time;154 short s_dinfo[4]; -- SystemV FS only155 unsigned long s_free_zones;156 unsigned short s_free_inodes;157 short s_dinfo[4]; -- Xenix FS only158 unsigned short s_interleave_m,s_interleave_n; -- Coherent FS only159 char s_fname[6];160 char s_fpack[6];161 162 then they differ considerably:163 164 Xenix FS::165 166 char s_clean;167 char s_fill[371];168 long s_magic;169 long s_type;170 171 SystemV FS::172 173 long s_fill[12 or 14];174 long s_state;175 long s_magic;176 long s_type;177 178 Coherent FS::179 180 unsigned long s_unique;181 182 Note that Coherent FS has no magic.183 184* Inode layout:185 186 - Minix FS::187 188 unsigned short i_mode;189 unsigned short i_uid;190 unsigned long i_size;191 unsigned long i_time;192 unsigned char i_gid;193 unsigned char i_nlinks;194 unsigned short i_zone[7+1+1];195 196 - Xenix FS, SystemV FS, Coherent FS::197 198 unsigned short i_mode;199 unsigned short i_nlink;200 unsigned short i_uid;201 unsigned short i_gid;202 unsigned long i_size;203 unsigned char i_zone[3*(10+1+1+1)];204 unsigned long i_atime;205 unsigned long i_mtime;206 unsigned long i_ctime;207 208 209* Regular file data blocks are organized as210 211 - Minix FS:212 213 - 7 direct blocks214 - 1 indirect block (pointers to blocks)215 - 1 double-indirect block (pointer to pointers to blocks)216 217 - Xenix FS, SystemV FS, Coherent FS:218 219 - 10 direct blocks220 - 1 indirect block (pointers to blocks)221 - 1 double-indirect block (pointer to pointers to blocks)222 - 1 triple-indirect block (pointer to pointers to pointers to blocks)223 224 225 =========== ========== ================226 Inode size inodes per block227 =========== ========== ================228 Minix FS 32 32229 Xenix FS 64 16230 SystemV FS 64 16231 Coherent FS 64 8232 =========== ========== ================233 234* Directory entry on disk235 236 - Minix FS::237 238 unsigned short inode;239 char name[14/30];240 241 - Xenix FS, SystemV FS, Coherent FS::242 243 unsigned short inode;244 char name[14];245 246 =========== ============== =====================247 Dir entry size dir entries per block248 =========== ============== =====================249 Minix FS 16/32 64/32250 Xenix FS 16 64251 SystemV FS 16 64252 Coherent FS 16 32253 =========== ============== =====================254 255* How to implement symbolic links such that the host fsck doesn't scream:256 257 - Minix FS normal258 - Xenix FS kludge: as regular files with chmod 1000259 - SystemV FS ??260 - Coherent FS kludge: as regular files with chmod 1000261 262 263Notation: We often speak of a "block" but mean a zone (the allocation unit)264and not the disk driver's notion of "block".265