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