1This is the implementation of the SystemV/Coherent filesystem for Linux. 2It 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 8and was merged together in July 1993. 9 10These 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 178Notation: We often speak of a "block" but mean a zone (the allocation unit) 179and not the disk driver's notion of "block". 180 181 182Bruno Haible <haible@ma2s2.mathematik.uni-karlsruhe.de> 183