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Change-Id: Ie94dbc1339facbee59080d85b6938f2c160eb05e Reviewed-on: https://review.haiku-os.org/c/haiku/+/7368 Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>
93 lines
3.6 KiB
ReStructuredText
93 lines
3.6 KiB
ReStructuredText
File systems overview
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---------------------
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Each filesystem driver must define a few structures which act as the
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interface between the VFS and the filesystem implementation. These
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structures contain function pointers, some of which are optional,
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which should point to functions defined by the implementer that
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perform the appropriate filesystem operation as defined by the
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documentation.
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See docs/user/drivers/fs_interface.dox for more detailed documentation
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of this interface.
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It's important to note that while there may be some similarities in
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the interface with that of other operations systems, one should not
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make any assumptions about the desired behavior based soley on the
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function prototypes defined in fs_interface.h.
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The following is a list of notes calling out some potential mistakes.
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# fs_vnode_ops.read_symlink
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Defining this function means that the filesystem driver supports
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symbolic links, and the function that f_vnode_ops.read_symlink points
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to should read the contents of a symlink from the specified node.
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This may seem similar to the posix function readlink(), but it is
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slightly different. Unlike readlink(), which returns the number of
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bytes copied into the output buffer, fs_vnode_ops.read_symlink is
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expected to always return the length of the symlink contents, even if
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the provided buffer is not large enough to contain the entire symlink
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contents.
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Development tools
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-----------------
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fs_shell
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........
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It is not convenient to test a filesystem by reloading its driver into a
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running Haiku system (kernel debugging is often not as easy as userland).
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Moreover, the filesystem interacts with other components of the system
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(file cache, block cache, but also any application reading or writing files).
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For the early development steps, it is much easier to run the filesystem code
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in a more controlled environment. This can be achieved through the use of
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a "filesystem shell": a simple application that runs the filesystem code, and
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allows performing specific operations through a command line interface. It can be
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executed on any platform (Haiku or another supported one such as Linux).
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Example of fs_shell implementations are available under src/tests/add-ons/kernel/file_systems/
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for the bfs and btrfs filesystems.
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For example, to build the fs_shell for btrfs, use
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.. code-block:: bash
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jam -q "<build>btrfs_shell"
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To run it, use
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.. code-block:: bash
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jam run ":<build>btrfs_shell" <.img file>
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:ref:`XFS Page` has a concrete example which can be tweaked for other filesystems.
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You need to pass at least a file or device containing a filesystem image as an
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argument. You need some tool to create one. It is possible to work using an
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actual disk volume (but be careful, it's risky to use one with useful data in it),
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a file, or a RAM disk, depending on what you are doing.
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userlandfs
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..........
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As a second step, it's possible to use the filesystem as part of a running
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system, while still running it in userland. This allows use of Debugger,
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memory protection, and in general any kind of userland debugging or tracing
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tool. When the filesystem crashes, it does not bring down the whole system.
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Userlandfs can run the filesystem code using the same interface as the kernel,
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therefore, once everything is working with userlandfs, running the filesystem
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as kernel code is usually quite easy (and provides a performance boost)
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See more here: :ref:`Userland FS Page`
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Torture and performance tests
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.............................
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Once the basic operations are working fine, it is a good idea to perform more
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agressive testing. Examples of scripts doing this are available in
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src/tests/add-ons/kernel/file_systems/ for the fat and ext2 filesystems.
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