Updated DESIGN.md to reflect v2 changes
Now with graphs! Images are stored on the branch gh-images in an effort to avoid binary bloat in the git history. Also spruced up SPEC.md and README.md and ran a spellechecker over the documentation. Favorite typo so far was dependendent, which is, in fact, not a word.
This commit is contained in:
Christopher Haster
@@ -1,6 +1,6 @@
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## The little filesystem
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## littlefs
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A little fail-safe filesystem designed for embedded systems.
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A little fail-safe filesystem designed for microcontrollers.
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```
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| | | .---._____
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@@ -11,17 +11,19 @@ A little fail-safe filesystem designed for embedded systems.
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```
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**Bounded RAM/ROM** - The littlefs is designed to work with a limited amount
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of memory. Recursion is avoided and dynamic memory is limited to configurable
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buffers that can be provided statically.
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**Power-loss resilience** - littlefs is designed to handle random power
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failures. All file operations have strong copy-on-write guarantees and if
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power is lost the filesystem will fall back to the last known good state.
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**Power-loss resilient** - The littlefs is designed for systems that may have
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random power failures. The littlefs has strong copy-on-write guarantees and
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storage on disk is always kept in a valid state.
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**Dynamic wear leveling** - littlefs is designed with flash in mind, and
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provides wear leveling over dynamic blocks. Additionally, littlefs can
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detect bad blocks and work around them.
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**Wear leveling** - Since the most common form of embedded storage is erodible
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flash memories, littlefs provides a form of dynamic wear leveling for systems
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that can not fit a full flash translation layer.
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**Bounded RAM/ROM** - littlefs is designed to work with a small amount of
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memory. RAM usage is strictly bounded, which means RAM consumption does not
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change as the filesystem grows. The filesystem contains no unbounded
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recursion and dynamic memory is limited to configurable buffers that can be
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provided statically.
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## Example
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@@ -91,11 +93,11 @@ int main(void) {
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Detailed documentation (or at least as much detail as is currently available)
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can be found in the comments in [lfs.h](lfs.h).
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As you may have noticed, littlefs takes in a configuration structure that
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defines how the filesystem operates. The configuration struct provides the
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filesystem with the block device operations and dimensions, tweakable
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parameters that tradeoff memory usage for performance, and optional
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static buffers if the user wants to avoid dynamic memory.
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littlefs takes in a configuration structure that defines how the filesystem
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operates. The configuration struct provides the filesystem with the block
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device operations and dimensions, tweakable parameters that tradeoff memory
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usage for performance, and optional static buffers if the user wants to avoid
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dynamic memory.
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The state of the littlefs is stored in the `lfs_t` type which is left up
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to the user to allocate, allowing multiple filesystems to be in use
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@@ -107,14 +109,14 @@ directory functions, with the deviation that the allocation of filesystem
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structures must be provided by the user.
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All POSIX operations, such as remove and rename, are atomic, even in event
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of power-loss. Additionally, no file updates are actually committed to the
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filesystem until sync or close is called on the file.
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of power-loss. Additionally, no file updates are not actually committed to
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the filesystem until sync or close is called on the file.
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## Other notes
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All littlefs have the potential to return a negative error code. The errors
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can be either one of those found in the `enum lfs_error` in [lfs.h](lfs.h),
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or an error returned by the user's block device operations.
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All littlefs functions have the potential to return a negative error code. The
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errors can be either one of those found in the `enum lfs_error` in
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[lfs.h](lfs.h), or an error returned by the user's block device operations.
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In the configuration struct, the `prog` and `erase` function provided by the
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user may return a `LFS_ERR_CORRUPT` error if the implementation already can
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@@ -128,14 +130,60 @@ from memory, otherwise data integrity can not be guaranteed. If the `write`
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function does not perform caching, and therefore each `read` or `write` call
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hits the memory, the `sync` function can simply return 0.
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## Reference material
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## Design
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[DESIGN.md](DESIGN.md) - DESIGN.md contains a fully detailed dive into how
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littlefs actually works. I would encourage you to read it since the
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solutions and tradeoffs at work here are quite interesting.
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At a high level, littlefs is a block based filesystem that uses small logs to
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store metadata and larger copy-on-write (COW) structures to store file data.
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[SPEC.md](SPEC.md) - SPEC.md contains the on-disk specification of littlefs
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with all the nitty-gritty details. Can be useful for developing tooling.
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In littlefs, these ingredients form a sort of two-layered cake, with the small
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logs (called metadata pairs) providing fast updates to metadata anywhere on
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storage, while the COW structures store file data compactly and without any
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wear amplification cost.
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Both of these data structures are built out of blocks, which are fed by a
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common block allocator. By limiting the number of erases allowed on a block
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per allocation, the allocator provides dynamic wear leveling over the entire
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filesystem.
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```
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root
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.--------.--------.
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| A'| B'| |
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| | |-> |
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| | | |
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'--------'--------'
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.----' '--------------.
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A v B v
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.--------.--------. .--------.--------.
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| C'| D'| | | E'|new| |
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| | |-> | | | E'|-> |
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| | | | | | | |
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'--------'--------' '--------'--------'
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.-' '--. | '------------------.
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v v .-' v
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.--------. .--------. v .--------.
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| C | | D | .--------. write | new E |
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| | | | | E | ==> | |
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| | | | | | | |
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'--------' '--------' | | '--------'
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'--------' .-' |
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.-' '-. .-------------|------'
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v v v v
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.--------. .--------. .--------.
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| F | | G | | new F |
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| | | | | |
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| | | | | |
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'--------' '--------' '--------'
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```
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More details on how littlefs works can be found in [DESIGN.md](DESIGN.md) and
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[SPEC.md](SPEC.md).
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- [DESIGN.md](DESIGN.md) - A fully detailed dive into how littlefs works.
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I would suggest reading it as the tradeoffs at work are quite interesting.
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- [SPEC.md](SPEC.md) - The on-disk specification of littlefs with all the
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nitty-gritty details. May be useful for tooling development.
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## Testing
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@@ -149,9 +197,9 @@ make test
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## License
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The littlefs is provided under the [BSD-3-Clause](https://spdx.org/licenses/BSD-3-Clause.html)
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license. See [LICENSE.md](LICENSE.md) for more information. Contributions to
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this project are accepted under the same license.
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The littlefs is provided under the [BSD-3-Clause] license. See
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[LICENSE.md](LICENSE.md) for more information. Contributions to this project
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are accepted under the same license.
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Individual files contain the following tag instead of the full license text.
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@@ -162,32 +210,39 @@ License Identifiers that are here available: http://spdx.org/licenses/
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## Related projects
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[Mbed OS](https://github.com/ARMmbed/mbed-os/tree/master/features/filesystem/littlefs) -
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The easiest way to get started with littlefs is to jump into [Mbed](https://os.mbed.com/),
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which already has block device drivers for most forms of embedded storage. The
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littlefs is available in Mbed OS as the [LittleFileSystem](https://os.mbed.com/docs/latest/reference/littlefilesystem.html)
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class.
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- [littlefs-fuse] - A [FUSE] wrapper for littlefs. The project allows you to
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mount littlefs directly on a Linux machine. Can be useful for debugging
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littlefs if you have an SD card handy.
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[littlefs-fuse](https://github.com/geky/littlefs-fuse) - A [FUSE](https://github.com/libfuse/libfuse)
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wrapper for littlefs. The project allows you to mount littlefs directly on a
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Linux machine. Can be useful for debugging littlefs if you have an SD card
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handy.
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- [littlefs-js] - A javascript wrapper for littlefs. I'm not sure why you would
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want this, but it is handy for demos. You can see it in action
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[here][littlefs-js-demo].
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[littlefs-js](https://github.com/geky/littlefs-js) - A javascript wrapper for
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littlefs. I'm not sure why you would want this, but it is handy for demos.
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You can see it in action [here](http://littlefs.geky.net/demo.html).
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- [mklfs] - A command line tool built by the [Lua RTOS] guys for making
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littlefs images from a host PC. Supports Windows, Mac OS, and Linux.
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[mklfs](https://github.com/whitecatboard/Lua-RTOS-ESP32/tree/master/components/mklfs/src) -
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A command line tool built by the [Lua RTOS](https://github.com/whitecatboard/Lua-RTOS-ESP32)
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guys for making littlefs images from a host PC. Supports Windows, Mac OS,
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and Linux.
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- [Mbed OS] - The easiest way to get started with littlefs is to jump into Mbed
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which already has block device drivers for most forms of embedded storage.
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littlefs is available in Mbed OS as the [LittleFileSystem] class.
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[SPIFFS](https://github.com/pellepl/spiffs) - Another excellent embedded
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filesystem for NOR flash. As a more traditional logging filesystem with full
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static wear-leveling, SPIFFS will likely outperform littlefs on small
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memories such as the internal flash on microcontrollers.
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- [SPIFFS] - Another excellent embedded filesystem for NOR flash. As a more
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traditional logging filesystem with full static wear-leveling, SPIFFS will
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likely outperform littlefs on small memories such as the internal flash on
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microcontrollers.
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[Dhara](https://github.com/dlbeer/dhara) - An interesting NAND flash
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translation layer designed for small MCUs. It offers static wear-leveling and
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power-resilience with only a fixed O(|address|) pointer structure stored on
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each block and in RAM.
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- [Dhara] - An interesting NAND flash translation layer designed for small
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MCUs. It offers static wear-leveling and power-resilience with only a fixed
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_O(|address|)_ pointer structure stored on each block and in RAM.
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[BSD-3-Clause]: https://spdx.org/licenses/BSD-3-Clause.html
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[littlefs-fuse]: https://github.com/geky/littlefs-fuse
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[FUSE]: https://github.com/libfuse/libfuse
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[littlefs-js]: https://github.com/geky/littlefs-js
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[littlefs-js-demo]:http://littlefs.geky.net/demo.html
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[mklfs]: https://github.com/whitecatboard/Lua-RTOS-ESP32/tree/master/components/mklfs/src
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[Lua RTOS]: https://github.com/whitecatboard/Lua-RTOS-ESP32
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[Mbed OS]: https://github.com/armmbed/mbed-os
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[LittleFileSystem]: https://os.mbed.com/docs/mbed-os/v5.12/apis/littlefilesystem.html
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[SPIFFS]: https://github.com/pellepl/spiffs
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[Dhara]: https://github.com/dlbeer/dhara
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