Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.
1 The QNX6 Filesystem 2 =================== 3 4 The qnx6fs is used by newer QNX operating system versions. (e.g. Neutrino) 5 It got introduced in QNX 6.4.0 and is used default since 6.4.1. 6 7 Option 8 ====== 9 10 mmi_fs Mount filesystem as used for example by Audi MMI 3G system 11 12 Specification 13 ============= 14 15 qnx6fs shares many properties with traditional Unix filesystems. It has the 16 concepts of blocks, inodes and directories. 17 On QNX it is possible to create little endian and big endian qnx6 filesystems. 18 This feature makes it possible to create and use a different endianness fs 19 for the target (QNX is used on quite a range of embedded systems) platform 20 running on a different endianness. 21 The Linux driver handles endianness transparently. (LE and BE) 22 23 Blocks 24 ------ 25 26 The space in the device or file is split up into blocks. These are a fixed 27 size of 512, 1024, 2048 or 4096, which is decided when the filesystem is 28 created. 29 Blockpointers are 32bit, so the maximum space that can be addressed is 30 2^32 * 4096 bytes or 16TB 31 32 The superblocks 33 --------------- 34 35 The superblock contains all global information about the filesystem. 36 Each qnx6fs got two superblocks, each one having a 64bit serial number. 37 That serial number is used to identify the "active" superblock. 38 In write mode with reach new snapshot (after each synchronous write), the 39 serial of the new master superblock is increased (old superblock serial + 1) 40 41 So basically the snapshot functionality is realized by an atomic final 42 update of the serial number. Before updating that serial, all modifications 43 are done by copying all modified blocks during that specific write request 44 (or period) and building up a new (stable) filesystem structure under the 45 inactive superblock. 46 47 Each superblock holds a set of root inodes for the different filesystem 48 parts. (Inode, Bitmap and Longfilenames) 49 Each of these root nodes holds information like total size of the stored 50 data and the addressing levels in that specific tree. 51 If the level value is 0, up to 16 direct blocks can be addressed by each 52 node. 53 Level 1 adds an additional indirect addressing level where each indirect 54 addressing block holds up to blocksize / 4 bytes pointers to data blocks. 55 Level 2 adds an additional indirect addressing block level (so, already up 56 to 16 * 256 * 256 = 1048576 blocks that can be addressed by such a tree). 57 58 Unused block pointers are always set to ~0 - regardless of root node, 59 indirect addressing blocks or inodes. 60 Data leaves are always on the lowest level. So no data is stored on upper 61 tree levels. 62 63 The first Superblock is located at 0x2000. (0x2000 is the bootblock size) 64 The Audi MMI 3G first superblock directly starts at byte 0. 65 Second superblock position can either be calculated from the superblock 66 information (total number of filesystem blocks) or by taking the highest 67 device address, zeroing the last 3 bytes and then subtracting 0x1000 from 68 that address. 69 70 0x1000 is the size reserved for each superblock - regardless of the 71 blocksize of the filesystem. 72 73 Inodes 74 ------ 75 76 Each object in the filesystem is represented by an inode. (index node) 77 The inode structure contains pointers to the filesystem blocks which contain 78 the data held in the object and all of the metadata about an object except 79 its longname. (filenames longer than 27 characters) 80 The metadata about an object includes the permissions, owner, group, flags, 81 size, number of blocks used, access time, change time and modification time. 82 83 Object mode field is POSIX format. (which makes things easier) 84 85 There are also pointers to the first 16 blocks, if the object data can be 86 addressed with 16 direct blocks. 87 For more than 16 blocks an indirect addressing in form of another tree is 88 used. (scheme is the same as the one used for the superblock root nodes) 89 90 The filesize is stored 64bit. Inode counting starts with 1. (whilst long 91 filename inodes start with 0) 92 93 Directories 94 ----------- 95 96 A directory is a filesystem object and has an inode just like a file. 97 It is a specially formatted file containing records which associate each 98 name with an inode number. 99 '.' inode number points to the directory inode 100 '..' inode number points to the parent directory inode 101 Eeach filename record additionally got a filename length field. 102 103 One special case are long filenames or subdirectory names. 104 These got set a filename length field of 0xff in the corresponding directory 105 record plus the longfile inode number also stored in that record. 106 With that longfilename inode number, the longfilename tree can be walked 107 starting with the superblock longfilename root node pointers. 108 109 Special files 110 ------------- 111 112 Symbolic links are also filesystem objects with inodes. They got a specific 113 bit in the inode mode field identifying them as symbolic link. 114 The directory entry file inode pointer points to the target file inode. 115 116 Hard links got an inode, a directory entry, but a specific mode bit set, 117 no block pointers and the directory file record pointing to the target file 118 inode. 119 120 Character and block special devices do not exist in QNX as those files 121 are handled by the QNX kernel/drivers and created in /dev independent of the 122 underlaying filesystem. 123 124 Long filenames 125 -------------- 126 127 Long filenames are stored in a separate addressing tree. The staring point 128 is the longfilename root node in the active superblock. 129 Each data block (tree leaves) holds one long filename. That filename is 130 limited to 510 bytes. The first two starting bytes are used as length field 131 for the actual filename. 132 If that structure shall fit for all allowed blocksizes, it is clear why there 133 is a limit of 510 bytes for the actual filename stored. 134 135 Bitmap 136 ------ 137 138 The qnx6fs filesystem allocation bitmap is stored in a tree under bitmap 139 root node in the superblock and each bit in the bitmap represents one 140 filesystem block. 141 The first block is block 0, which starts 0x1000 after superblock start. 142 So for a normal qnx6fs 0x3000 (bootblock + superblock) is the physical 143 address at which block 0 is located. 144 145 Bits at the end of the last bitmap block are set to 1, if the device is 146 smaller than addressing space in the bitmap. 147 148 Bitmap system area 149 ------------------ 150 151 The bitmap itself is divided into three parts. 152 First the system area, that is split into two halves. 153 Then userspace. 154 155 The requirement for a static, fixed preallocated system area comes from how 156 qnx6fs deals with writes. 157 Each superblock got it's own half of the system area. So superblock #1 158 always uses blocks from the lower half whilst superblock #2 just writes to 159 blocks represented by the upper half bitmap system area bits. 160 161 Bitmap blocks, Inode blocks and indirect addressing blocks for those two 162 tree structures are treated as system blocks. 163 164 The rational behind that is that a write request can work on a new snapshot 165 (system area of the inactive - resp. lower serial numbered superblock) while 166 at the same time there is still a complete stable filesystem structer in the 167 other half of the system area. 168 169 When finished with writing (a sync write is completed, the maximum sync leap 170 time or a filesystem sync is requested), serial of the previously inactive 171 superblock atomically is increased and the fs switches over to that - then 172 stable declared - superblock. 173 174 For all data outside the system area, blocks are just copied while writing.