diff options
Diffstat (limited to 'Documentation/filesystems/qnx6.txt')
| -rw-r--r-- | Documentation/filesystems/qnx6.txt | 174 |
1 files changed, 0 insertions, 174 deletions
diff --git a/Documentation/filesystems/qnx6.txt b/Documentation/filesystems/qnx6.txt deleted file mode 100644 index 48ea68f15845..000000000000 --- a/Documentation/filesystems/qnx6.txt +++ /dev/null @@ -1,174 +0,0 @@ -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 -underlaying 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 it's 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 structer 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. |