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diff --git a/Documentation/filesystems/qnx6.rst b/Documentation/filesystems/qnx6.rst new file mode 100644 index 000000000000..560f3d470422 --- /dev/null +++ b/Documentation/filesystems/qnx6.rst @@ -0,0 +1,196 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=================== +The QNX6 Filesystem +=================== + +The qnx6fs is used by newer QNX operating system versions. (e.g. Neutrino) +It got introduced in QNX 6.4.0 and is used default since 6.4.1. + +Option +====== + +mmi_fs Mount filesystem as used for example by Audi MMI 3G system + +Specification +============= + +qnx6fs shares many properties with traditional Unix filesystems. It has the +concepts of blocks, inodes and directories. + +On QNX it is possible to create little endian and big endian qnx6 filesystems. +This feature makes it possible to create and use a different endianness fs +for the target (QNX is used on quite a range of embedded systems) platform +running on a different endianness. + +The Linux driver handles endianness transparently. (LE and BE) + +Blocks +------ + +The space in the device or file is split up into blocks. These are a fixed +size of 512, 1024, 2048 or 4096, which is decided when the filesystem is +created. + +Blockpointers are 32bit, so the maximum space that can be addressed is +2^32 * 4096 bytes or 16TB + +The superblocks +--------------- + +The superblock contains all global information about the filesystem. +Each qnx6fs got two superblocks, each one having a 64bit serial number. +That serial number is used to identify the "active" superblock. +In write mode with reach new snapshot (after each synchronous write), the +serial of the new master superblock is increased (old superblock serial + 1) + +So basically the snapshot functionality is realized by an atomic final +update of the serial number. Before updating that serial, all modifications +are done by copying all modified blocks during that specific write request +(or period) and building up a new (stable) filesystem structure under the +inactive superblock. + +Each superblock holds a set of root inodes for the different filesystem +parts. (Inode, Bitmap and Longfilenames) +Each of these root nodes holds information like total size of the stored +data and the addressing levels in that specific tree. +If the level value is 0, up to 16 direct blocks can be addressed by each +node. + +Level 1 adds an additional indirect addressing level where each indirect +addressing block holds up to blocksize / 4 bytes pointers to data blocks. +Level 2 adds an additional indirect addressing block level (so, already up +to 16 * 256 * 256 = 1048576 blocks that can be addressed by such a tree). + +Unused block pointers are always set to ~0 - regardless of root node, +indirect addressing blocks or inodes. + +Data leaves are always on the lowest level. So no data is stored on upper +tree levels. + +The first Superblock is located at 0x2000. (0x2000 is the bootblock size) +The Audi MMI 3G first superblock directly starts at byte 0. + +Second superblock position can either be calculated from the superblock +information (total number of filesystem blocks) or by taking the highest +device address, zeroing the last 3 bytes and then subtracting 0x1000 from +that address. + +0x1000 is the size reserved for each superblock - regardless of the +blocksize of the filesystem. + +Inodes +------ + +Each object in the filesystem is represented by an inode. (index node) +The inode structure contains pointers to the filesystem blocks which contain +the data held in the object and all of the metadata about an object except +its longname. (filenames longer than 27 characters) +The metadata about an object includes the permissions, owner, group, flags, +size, number of blocks used, access time, change time and modification time. + +Object mode field is POSIX format. (which makes things easier) + +There are also pointers to the first 16 blocks, if the object data can be +addressed with 16 direct blocks. + +For more than 16 blocks an indirect addressing in form of another tree is +used. (scheme is the same as the one used for the superblock root nodes) + +The filesize is stored 64bit. Inode counting starts with 1. (while long +filename inodes start with 0) + +Directories +----------- + +A directory is a filesystem object and has an inode just like a file. +It is a specially formatted file containing records which associate each +name with an inode number. + +'.' inode number points to the directory inode + +'..' inode number points to the parent directory inode + +Eeach filename record additionally got a filename length field. + +One special case are long filenames or subdirectory names. + +These got set a filename length field of 0xff in the corresponding directory +record plus the longfile inode number also stored in that record. + +With that longfilename inode number, the longfilename tree can be walked +starting with the superblock longfilename root node pointers. + +Special files +------------- + +Symbolic links are also filesystem objects with inodes. They got a specific +bit in the inode mode field identifying them as symbolic link. + +The directory entry file inode pointer points to the target file inode. + +Hard links got an inode, a directory entry, but a specific mode bit set, +no block pointers and the directory file record pointing to the target file +inode. + +Character and block special devices do not exist in QNX as those files +are handled by the QNX kernel/drivers and created in /dev independent of the +underlying filesystem. + +Long filenames +-------------- + +Long filenames are stored in a separate addressing tree. The staring point +is the longfilename root node in the active superblock. + +Each data block (tree leaves) holds one long filename. That filename is +limited to 510 bytes. The first two starting bytes are used as length field +for the actual filename. + +If that structure shall fit for all allowed blocksizes, it is clear why there +is a limit of 510 bytes for the actual filename stored. + +Bitmap +------ + +The qnx6fs filesystem allocation bitmap is stored in a tree under bitmap +root node in the superblock and each bit in the bitmap represents one +filesystem block. + +The first block is block 0, which starts 0x1000 after superblock start. +So for a normal qnx6fs 0x3000 (bootblock + superblock) is the physical +address at which block 0 is located. + +Bits at the end of the last bitmap block are set to 1, if the device is +smaller than addressing space in the bitmap. + +Bitmap system area +------------------ + +The bitmap itself is divided into three parts. + +First the system area, that is split into two halves. + +Then userspace. + +The requirement for a static, fixed preallocated system area comes from how +qnx6fs deals with writes. + +Each superblock got its own half of the system area. So superblock #1 +always uses blocks from the lower half while superblock #2 just writes to +blocks represented by the upper half bitmap system area bits. + +Bitmap blocks, Inode blocks and indirect addressing blocks for those two +tree structures are treated as system blocks. + +The rational behind that is that a write request can work on a new snapshot +(system area of the inactive - resp. lower serial numbered superblock) while +at the same time there is still a complete stable filesystem structure in the +other half of the system area. + +When finished with writing (a sync write is completed, the maximum sync leap +time or a filesystem sync is requested), serial of the previously inactive +superblock atomically is increased and the fs switches over to that - then +stable declared - superblock. + +For all data outside the system area, blocks are just copied while writing. |