1 .. SPDX-License-Identifier: GPL-2.0 2 3 =================== 4 The QNX6 Filesystem 5 =================== 6 7 The qnx6fs is used by newer QNX operating system versions. (e.g. Neutrino) 8 It got introduced in QNX 6.4.0 and is used default since 6.4.1. 9 10 Option 11 ====== 12 13 mmi_fs Mount filesystem as used for example by Audi MMI 3G system 14 15 Specification 16 ============= 17 18 qnx6fs shares many properties with traditional Unix filesystems. It has the 19 concepts of blocks, inodes and directories. 20 21 On QNX it is possible to create little endian and big endian qnx6 filesystems. 22 This feature makes it possible to create and use a different endianness fs 23 for the target (QNX is used on quite a range of embedded systems) platform 24 running on a different endianness. 25 26 The Linux driver handles endianness transparently. (LE and BE) 27 28 Blocks 29 ------ 30 31 The space in the device or file is split up into blocks. These are a fixed 32 size of 512, 1024, 2048 or 4096, which is decided when the filesystem is 33 created. 34 35 Blockpointers are 32bit, so the maximum space that can be addressed is 36 2^32 * 4096 bytes or 16TB 37 38 The superblocks 39 --------------- 40 41 The superblock contains all global information about the filesystem. 42 Each qnx6fs got two superblocks, each one having a 64bit serial number. 43 That serial number is used to identify the "active" superblock. 44 In write mode with reach new snapshot (after each synchronous write), the 45 serial of the new master superblock is increased (old superblock serial + 1) 46 47 So basically the snapshot functionality is realized by an atomic final 48 update of the serial number. Before updating that serial, all modifications 49 are done by copying all modified blocks during that specific write request 50 (or period) and building up a new (stable) filesystem structure under the 51 inactive superblock. 52 53 Each superblock holds a set of root inodes for the different filesystem 54 parts. (Inode, Bitmap and Longfilenames) 55 Each of these root nodes holds information like total size of the stored 56 data and the addressing levels in that specific tree. 57 If the level value is 0, up to 16 direct blocks can be addressed by each 58 node. 59 60 Level 1 adds an additional indirect addressing level where each indirect 61 addressing block holds up to blocksize / 4 bytes pointers to data blocks. 62 Level 2 adds an additional indirect addressing block level (so, already up 63 to 16 * 256 * 256 = 1048576 blocks that can be addressed by such a tree). 64 65 Unused block pointers are always set to ~0 - regardless of root node, 66 indirect addressing blocks or inodes. 67 68 Data leaves are always on the lowest level. So no data is stored on upper 69 tree levels. 70 71 The first Superblock is located at 0x2000. (0x2000 is the bootblock size) 72 The Audi MMI 3G first superblock directly starts at byte 0. 73 74 Second superblock position can either be calculated from the superblock 75 information (total number of filesystem blocks) or by taking the highest 76 device address, zeroing the last 3 bytes and then subtracting 0x1000 from 77 that address. 78 79 0x1000 is the size reserved for each superblock - regardless of the 80 blocksize of the filesystem. 81 82 Inodes 83 ------ 84 85 Each object in the filesystem is represented by an inode. (index node) 86 The inode structure contains pointers to the filesystem blocks which contain 87 the data held in the object and all of the metadata about an object except 88 its longname. (filenames longer than 27 characters) 89 The metadata about an object includes the permissions, owner, group, flags, 90 size, number of blocks used, access time, change time and modification time. 91 92 Object mode field is POSIX format. (which makes things easier) 93 94 There are also pointers to the first 16 blocks, if the object data can be 95 addressed with 16 direct blocks. 96 97 For more than 16 blocks an indirect addressing in form of another tree is 98 used. (scheme is the same as the one used for the superblock root nodes) 99 100 The filesize is stored 64bit. Inode counting starts with 1. (while long 101 filename inodes start with 0) 102 103 Directories 104 ----------- 105 106 A directory is a filesystem object and has an inode just like a file. 107 It is a specially formatted file containing records which associate each 108 name with an inode number. 109 110 '.' inode number points to the directory inode 111 112 '..' inode number points to the parent directory inode 113 114 Eeach filename record additionally got a filename length field. 115 116 One special case are long filenames or subdirectory names. 117 118 These got set a filename length field of 0xff in the corresponding directory 119 record plus the longfile inode number also stored in that record. 120 121 With that longfilename inode number, the longfilename tree can be walked 122 starting with the superblock longfilename root node pointers. 123 124 Special files 125 ------------- 126 127 Symbolic links are also filesystem objects with inodes. They got a specific 128 bit in the inode mode field identifying them as symbolic link. 129 130 The directory entry file inode pointer points to the target file inode. 131 132 Hard links got an inode, a directory entry, but a specific mode bit set, 133 no block pointers and the directory file record pointing to the target file 134 inode. 135 136 Character and block special devices do not exist in QNX as those files 137 are handled by the QNX kernel/drivers and created in /dev independent of the 138 underlying filesystem. 139 140 Long filenames 141 -------------- 142 143 Long filenames are stored in a separate addressing tree. The staring point 144 is the longfilename root node in the active superblock. 145 146 Each data block (tree leaves) holds one long filename. That filename is 147 limited to 510 bytes. The first two starting bytes are used as length field 148 for the actual filename. 149 150 If that structure shall fit for all allowed blocksizes, it is clear why there 151 is a limit of 510 bytes for the actual filename stored. 152 153 Bitmap 154 ------ 155 156 The qnx6fs filesystem allocation bitmap is stored in a tree under bitmap 157 root node in the superblock and each bit in the bitmap represents one 158 filesystem block. 159 160 The first block is block 0, which starts 0x1000 after superblock start. 161 So for a normal qnx6fs 0x3000 (bootblock + superblock) is the physical 162 address at which block 0 is located. 163 164 Bits at the end of the last bitmap block are set to 1, if the device is 165 smaller than addressing space in the bitmap. 166 167 Bitmap system area 168 ------------------ 169 170 The bitmap itself is divided into three parts. 171 172 First the system area, that is split into two halves. 173 174 Then userspace. 175 176 The requirement for a static, fixed preallocated system area comes from how 177 qnx6fs deals with writes. 178 179 Each superblock got its own half of the system area. So superblock #1 180 always uses blocks from the lower half while superblock #2 just writes to 181 blocks represented by the upper half bitmap system area bits. 182 183 Bitmap blocks, Inode blocks and indirect addressing blocks for those two 184 tree structures are treated as system blocks. 185 186 The rational behind that is that a write request can work on a new snapshot 187 (system area of the inactive - resp. lower serial numbered superblock) while 188 at the same time there is still a complete stable filesystem structure in the 189 other half of the system area. 190 191 When finished with writing (a sync write is completed, the maximum sync leap 192 time or a filesystem sync is requested), serial of the previously inactive 193 superblock atomically is increased and the fs switches over to that - then 194 stable declared - superblock. 195 196 For all data outside the system area, blocks are just copied while writing.
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