1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 =================== 3 ===================
4 The QNX6 Filesystem 4 The QNX6 Filesystem
5 =================== 5 ===================
6 6
7 The qnx6fs is used by newer QNX operating syst 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 def 8 It got introduced in QNX 6.4.0 and is used default since 6.4.1.
9 9
10 Option 10 Option
11 ====== 11 ======
12 12
13 mmi_fs Mount filesystem as used for e 13 mmi_fs Mount filesystem as used for example by Audi MMI 3G system
14 14
15 Specification 15 Specification
16 ============= 16 =============
17 17
18 qnx6fs shares many properties with traditional 18 qnx6fs shares many properties with traditional Unix filesystems. It has the
19 concepts of blocks, inodes and directories. 19 concepts of blocks, inodes and directories.
20 20
21 On QNX it is possible to create little endian 21 On QNX it is possible to create little endian and big endian qnx6 filesystems.
22 This feature makes it possible to create and u 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 o 23 for the target (QNX is used on quite a range of embedded systems) platform
24 running on a different endianness. 24 running on a different endianness.
25 25
26 The Linux driver handles endianness transparen 26 The Linux driver handles endianness transparently. (LE and BE)
27 27
28 Blocks 28 Blocks
29 ------ 29 ------
30 30
31 The space in the device or file is split up in 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 deci 32 size of 512, 1024, 2048 or 4096, which is decided when the filesystem is
33 created. 33 created.
34 34
35 Blockpointers are 32bit, so the maximum space 35 Blockpointers are 32bit, so the maximum space that can be addressed is
36 2^32 * 4096 bytes or 16TB 36 2^32 * 4096 bytes or 16TB
37 37
38 The superblocks 38 The superblocks
39 --------------- 39 ---------------
40 40
41 The superblock contains all global information 41 The superblock contains all global information about the filesystem.
42 Each qnx6fs got two superblocks, each one havi 42 Each qnx6fs got two superblocks, each one having a 64bit serial number.
43 That serial number is used to identify the "ac 43 That serial number is used to identify the "active" superblock.
44 In write mode with reach new snapshot (after e 44 In write mode with reach new snapshot (after each synchronous write), the
45 serial of the new master superblock is increas 45 serial of the new master superblock is increased (old superblock serial + 1)
46 46
47 So basically the snapshot functionality is rea 47 So basically the snapshot functionality is realized by an atomic final
48 update of the serial number. Before updating t 48 update of the serial number. Before updating that serial, all modifications
49 are done by copying all modified blocks during 49 are done by copying all modified blocks during that specific write request
50 (or period) and building up a new (stable) fil 50 (or period) and building up a new (stable) filesystem structure under the
51 inactive superblock. 51 inactive superblock.
52 52
53 Each superblock holds a set of root inodes for 53 Each superblock holds a set of root inodes for the different filesystem
54 parts. (Inode, Bitmap and Longfilenames) 54 parts. (Inode, Bitmap and Longfilenames)
55 Each of these root nodes holds information lik 55 Each of these root nodes holds information like total size of the stored
56 data and the addressing levels in that specifi 56 data and the addressing levels in that specific tree.
57 If the level value is 0, up to 16 direct block 57 If the level value is 0, up to 16 direct blocks can be addressed by each
58 node. 58 node.
59 59
60 Level 1 adds an additional indirect addressing 60 Level 1 adds an additional indirect addressing level where each indirect
61 addressing block holds up to blocksize / 4 byt 61 addressing block holds up to blocksize / 4 bytes pointers to data blocks.
62 Level 2 adds an additional indirect addressing 62 Level 2 adds an additional indirect addressing block level (so, already up
63 to 16 * 256 * 256 = 1048576 blocks that can be 63 to 16 * 256 * 256 = 1048576 blocks that can be addressed by such a tree).
64 64
65 Unused block pointers are always set to ~0 - r 65 Unused block pointers are always set to ~0 - regardless of root node,
66 indirect addressing blocks or inodes. 66 indirect addressing blocks or inodes.
67 67
68 Data leaves are always on the lowest level. So 68 Data leaves are always on the lowest level. So no data is stored on upper
69 tree levels. 69 tree levels.
70 70
71 The first Superblock is located at 0x2000. (0x 71 The first Superblock is located at 0x2000. (0x2000 is the bootblock size)
72 The Audi MMI 3G first superblock directly star 72 The Audi MMI 3G first superblock directly starts at byte 0.
73 73
74 Second superblock position can either be calcu 74 Second superblock position can either be calculated from the superblock
75 information (total number of filesystem blocks 75 information (total number of filesystem blocks) or by taking the highest
76 device address, zeroing the last 3 bytes and t 76 device address, zeroing the last 3 bytes and then subtracting 0x1000 from
77 that address. 77 that address.
78 78
79 0x1000 is the size reserved for each superbloc 79 0x1000 is the size reserved for each superblock - regardless of the
80 blocksize of the filesystem. 80 blocksize of the filesystem.
81 81
82 Inodes 82 Inodes
83 ------ 83 ------
84 84
85 Each object in the filesystem is represented b 85 Each object in the filesystem is represented by an inode. (index node)
86 The inode structure contains pointers to the f 86 The inode structure contains pointers to the filesystem blocks which contain
87 the data held in the object and all of the met 87 the data held in the object and all of the metadata about an object except
88 its longname. (filenames longer than 27 charac 88 its longname. (filenames longer than 27 characters)
89 The metadata about an object includes the perm 89 The metadata about an object includes the permissions, owner, group, flags,
90 size, number of blocks used, access time, chan 90 size, number of blocks used, access time, change time and modification time.
91 91
92 Object mode field is POSIX format. (which make 92 Object mode field is POSIX format. (which makes things easier)
93 93
94 There are also pointers to the first 16 blocks 94 There are also pointers to the first 16 blocks, if the object data can be
95 addressed with 16 direct blocks. 95 addressed with 16 direct blocks.
96 96
97 For more than 16 blocks an indirect addressing 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 98 used. (scheme is the same as the one used for the superblock root nodes)
99 99
100 The filesize is stored 64bit. Inode counting s 100 The filesize is stored 64bit. Inode counting starts with 1. (while long
101 filename inodes start with 0) 101 filename inodes start with 0)
102 102
103 Directories 103 Directories
104 ----------- 104 -----------
105 105
106 A directory is a filesystem object and has an 106 A directory is a filesystem object and has an inode just like a file.
107 It is a specially formatted file containing re 107 It is a specially formatted file containing records which associate each
108 name with an inode number. 108 name with an inode number.
109 109
110 '.' inode number points to the directory inode 110 '.' inode number points to the directory inode
111 111
112 '..' inode number points to the parent directo 112 '..' inode number points to the parent directory inode
113 113
114 Eeach filename record additionally got a filen 114 Eeach filename record additionally got a filename length field.
115 115
116 One special case are long filenames or subdire 116 One special case are long filenames or subdirectory names.
117 117
118 These got set a filename length field of 0xff 118 These got set a filename length field of 0xff in the corresponding directory
119 record plus the longfile inode number also sto 119 record plus the longfile inode number also stored in that record.
120 120
121 With that longfilename inode number, the longf 121 With that longfilename inode number, the longfilename tree can be walked
122 starting with the superblock longfilename root 122 starting with the superblock longfilename root node pointers.
123 123
124 Special files 124 Special files
125 ------------- 125 -------------
126 126
127 Symbolic links are also filesystem objects wit 127 Symbolic links are also filesystem objects with inodes. They got a specific
128 bit in the inode mode field identifying them a 128 bit in the inode mode field identifying them as symbolic link.
129 129
130 The directory entry file inode pointer points 130 The directory entry file inode pointer points to the target file inode.
131 131
132 Hard links got an inode, a directory entry, bu 132 Hard links got an inode, a directory entry, but a specific mode bit set,
133 no block pointers and the directory file recor 133 no block pointers and the directory file record pointing to the target file
134 inode. 134 inode.
135 135
136 Character and block special devices do not exi 136 Character and block special devices do not exist in QNX as those files
137 are handled by the QNX kernel/drivers and crea 137 are handled by the QNX kernel/drivers and created in /dev independent of the
138 underlying filesystem. !! 138 underlaying filesystem.
139 139
140 Long filenames 140 Long filenames
141 -------------- 141 --------------
142 142
143 Long filenames are stored in a separate addres 143 Long filenames are stored in a separate addressing tree. The staring point
144 is the longfilename root node in the active su 144 is the longfilename root node in the active superblock.
145 145
146 Each data block (tree leaves) holds one long f 146 Each data block (tree leaves) holds one long filename. That filename is
147 limited to 510 bytes. The first two starting b 147 limited to 510 bytes. The first two starting bytes are used as length field
148 for the actual filename. 148 for the actual filename.
149 149
150 If that structure shall fit for all allowed bl 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 filenam 151 is a limit of 510 bytes for the actual filename stored.
152 152
153 Bitmap 153 Bitmap
154 ------ 154 ------
155 155
156 The qnx6fs filesystem allocation bitmap is sto 156 The qnx6fs filesystem allocation bitmap is stored in a tree under bitmap
157 root node in the superblock and each bit in th 157 root node in the superblock and each bit in the bitmap represents one
158 filesystem block. 158 filesystem block.
159 159
160 The first block is block 0, which starts 0x100 160 The first block is block 0, which starts 0x1000 after superblock start.
161 So for a normal qnx6fs 0x3000 (bootblock + sup 161 So for a normal qnx6fs 0x3000 (bootblock + superblock) is the physical
162 address at which block 0 is located. 162 address at which block 0 is located.
163 163
164 Bits at the end of the last bitmap block are s 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. 165 smaller than addressing space in the bitmap.
166 166
167 Bitmap system area 167 Bitmap system area
168 ------------------ 168 ------------------
169 169
170 The bitmap itself is divided into three parts. 170 The bitmap itself is divided into three parts.
171 171
172 First the system area, that is split into two 172 First the system area, that is split into two halves.
173 173
174 Then userspace. 174 Then userspace.
175 175
176 The requirement for a static, fixed preallocat 176 The requirement for a static, fixed preallocated system area comes from how
177 qnx6fs deals with writes. 177 qnx6fs deals with writes.
178 178
179 Each superblock got its own half of the system 179 Each superblock got its own half of the system area. So superblock #1
180 always uses blocks from the lower half while s 180 always uses blocks from the lower half while superblock #2 just writes to
181 blocks represented by the upper half bitmap sy 181 blocks represented by the upper half bitmap system area bits.
182 182
183 Bitmap blocks, Inode blocks and indirect addre 183 Bitmap blocks, Inode blocks and indirect addressing blocks for those two
184 tree structures are treated as system blocks. 184 tree structures are treated as system blocks.
185 185
186 The rational behind that is that a write reque 186 The rational behind that is that a write request can work on a new snapshot
187 (system area of the inactive - resp. lower ser 187 (system area of the inactive - resp. lower serial numbered superblock) while
188 at the same time there is still a complete sta 188 at the same time there is still a complete stable filesystem structure in the
189 other half of the system area. 189 other half of the system area.
190 190
191 When finished with writing (a sync write is co 191 When finished with writing (a sync write is completed, the maximum sync leap
192 time or a filesystem sync is requested), seria 192 time or a filesystem sync is requested), serial of the previously inactive
193 superblock atomically is increased and the fs 193 superblock atomically is increased and the fs switches over to that - then
194 stable declared - superblock. 194 stable declared - superblock.
195 195
196 For all data outside the system area, blocks a 196 For all data outside the system area, blocks are just copied while writing.
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