1## littlefs
2
3A little fail-safe filesystem designed for microcontrollers.
4
5```
6   | | |     .---._____
7  .-----.   |          |
8--|o    |---| littlefs |
9--|     |---|          |
10  '-----'   '----------'
11   | | |
12```
13
14**Power-loss resilience** - littlefs is designed to handle random power
15failures. All file operations have strong copy-on-write guarantees and if
16power is lost the filesystem will fall back to the last known good state.
17
18**Dynamic wear leveling** - littlefs is designed with flash in mind, and
19provides wear leveling over dynamic blocks. Additionally, littlefs can
20detect bad blocks and work around them.
21
22**Bounded RAM/ROM** - littlefs is designed to work with a small amount of
23memory. RAM usage is strictly bounded, which means RAM consumption does not
24change as the filesystem grows. The filesystem contains no unbounded
25recursion and dynamic memory is limited to configurable buffers that can be
26provided statically.
27
28## Example
29
30Here's a simple example that updates a file named `boot_count` every time
31main runs. The program can be interrupted at any time without losing track
32of how many times it has been booted and without corrupting the filesystem:
33
34``` c
35#include "lfs.h"
36
37// variables used by the filesystem
38lfs_t lfs;
39lfs_file_t file;
40
41// configuration of the filesystem is provided by this struct
42const struct lfs_config cfg = {
43    // block device operations
44    .read  = user_provided_block_device_read,
45    .prog  = user_provided_block_device_prog,
46    .erase = user_provided_block_device_erase,
47    .sync  = user_provided_block_device_sync,
48
49    // block device configuration
50    .read_size = 16,
51    .prog_size = 16,
52    .block_size = 4096,
53    .block_count = 128,
54    .cache_size = 16,
55    .lookahead_size = 16,
56    .block_cycles = 500,
57};
58
59// entry point
60int main(void) {
61    // mount the filesystem
62    int err = lfs_mount(&lfs, &cfg);
63
64    // reformat if we can't mount the filesystem
65    // this should only happen on the first boot
66    if (err) {
67        lfs_format(&lfs, &cfg);
68        lfs_mount(&lfs, &cfg);
69    }
70
71    // read current count
72    uint32_t boot_count = 0;
73    lfs_file_open(&lfs, &file, "boot_count", LFS_O_RDWR | LFS_O_CREAT);
74    lfs_file_read(&lfs, &file, &boot_count, sizeof(boot_count));
75
76    // update boot count
77    boot_count += 1;
78    lfs_file_rewind(&lfs, &file);
79    lfs_file_write(&lfs, &file, &boot_count, sizeof(boot_count));
80
81    // remember the storage is not updated until the file is closed successfully
82    lfs_file_close(&lfs, &file);
83
84    // release any resources we were using
85    lfs_unmount(&lfs);
86
87    // print the boot count
88    printf("boot_count: %d\n", boot_count);
89}
90```
91
92## Usage
93
94Detailed documentation (or at least as much detail as is currently available)
95can be found in the comments in [lfs.h](lfs.h).
96
97littlefs takes in a configuration structure that defines how the filesystem
98operates. The configuration struct provides the filesystem with the block
99device operations and dimensions, tweakable parameters that tradeoff memory
100usage for performance, and optional static buffers if the user wants to avoid
101dynamic memory.
102
103The state of the littlefs is stored in the `lfs_t` type which is left up
104to the user to allocate, allowing multiple filesystems to be in use
105simultaneously. With the `lfs_t` and configuration struct, a user can
106format a block device or mount the filesystem.
107
108Once mounted, the littlefs provides a full set of POSIX-like file and
109directory functions, with the deviation that the allocation of filesystem
110structures must be provided by the user.
111
112All POSIX operations, such as remove and rename, are atomic, even in event
113of power-loss. Additionally, file updates are not actually committed to
114the filesystem until sync or close is called on the file.
115
116## Other notes
117
118Littlefs is written in C, and specifically should compile with any compiler
119that conforms to the `C99` standard.
120
121All littlefs calls have the potential to return a negative error code. The
122errors can be either one of those found in the `enum lfs_error` in
123[lfs.h](lfs.h), or an error returned by the user's block device operations.
124
125In the configuration struct, the `prog` and `erase` function provided by the
126user may return a `LFS_ERR_CORRUPT` error if the implementation already can
127detect corrupt blocks. However, the wear leveling does not depend on the return
128code of these functions, instead all data is read back and checked for
129integrity.
130
131If your storage caches writes, make sure that the provided `sync` function
132flushes all the data to memory and ensures that the next read fetches the data
133from memory, otherwise data integrity can not be guaranteed. If the `write`
134function does not perform caching, and therefore each `read` or `write` call
135hits the memory, the `sync` function can simply return 0.
136
137## Design
138
139At a high level, littlefs is a block based filesystem that uses small logs to
140store metadata and larger copy-on-write (COW) structures to store file data.
141
142In littlefs, these ingredients form a sort of two-layered cake, with the small
143logs (called metadata pairs) providing fast updates to metadata anywhere on
144storage, while the COW structures store file data compactly and without any
145wear amplification cost.
146
147Both of these data structures are built out of blocks, which are fed by a
148common block allocator. By limiting the number of erases allowed on a block
149per allocation, the allocator provides dynamic wear leveling over the entire
150filesystem.
151
152```
153                    root
154                   .--------.--------.
155                   | A'| B'|         |
156                   |   |   |->       |
157                   |   |   |         |
158                   '--------'--------'
159                .----'   '--------------.
160       A       v                 B       v
161      .--------.--------.       .--------.--------.
162      | C'| D'|         |       | E'|new|         |
163      |   |   |->       |       |   | E'|->       |
164      |   |   |         |       |   |   |         |
165      '--------'--------'       '--------'--------'
166      .-'   '--.                  |   '------------------.
167     v          v              .-'                        v
168.--------.  .--------.        v                       .--------.
169|   C    |  |   D    |   .--------.       write       | new E  |
170|        |  |        |   |   E    |        ==>        |        |
171|        |  |        |   |        |                   |        |
172'--------'  '--------'   |        |                   '--------'
173                         '--------'                   .-'    |
174                         .-'    '-.    .-------------|------'
175                        v          v  v              v
176                   .--------.  .--------.       .--------.
177                   |   F    |  |   G    |       | new F  |
178                   |        |  |        |       |        |
179                   |        |  |        |       |        |
180                   '--------'  '--------'       '--------'
181```
182
183More details on how littlefs works can be found in [DESIGN.md](DESIGN.md) and
184[SPEC.md](SPEC.md).
185
186- [DESIGN.md](DESIGN.md) - A fully detailed dive into how littlefs works.
187  I would suggest reading it as the tradeoffs at work are quite interesting.
188
189- [SPEC.md](SPEC.md) - The on-disk specification of littlefs with all the
190  nitty-gritty details. May be useful for tooling development.
191
192## Testing
193
194The littlefs comes with a test suite designed to run on a PC using the
195[emulated block device](emubd/lfs_emubd.h) found in the emubd directory.
196The tests assume a Linux environment and can be started with make:
197
198``` bash
199make test
200```
201
202## License
203
204The littlefs is provided under the [BSD-3-Clause] license. See
205[LICENSE.md](LICENSE.md) for more information. Contributions to this project
206are accepted under the same license.
207
208Individual files contain the following tag instead of the full license text.
209
210    SPDX-License-Identifier:    BSD-3-Clause
211
212This enables machine processing of license information based on the SPDX
213License Identifiers that are here available: http://spdx.org/licenses/
214
215## Related projects
216
217- [littlefs-fuse] - A [FUSE] wrapper for littlefs. The project allows you to
218  mount littlefs directly on a Linux machine. Can be useful for debugging
219  littlefs if you have an SD card handy.
220
221- [littlefs-js] - A javascript wrapper for littlefs. I'm not sure why you would
222  want this, but it is handy for demos.  You can see it in action
223  [here][littlefs-js-demo].
224
225- [mklfs] - A command line tool built by the [Lua RTOS] guys for making
226  littlefs images from a host PC. Supports Windows, Mac OS, and Linux.
227
228- [Mbed OS] - The easiest way to get started with littlefs is to jump into Mbed
229  which already has block device drivers for most forms of embedded storage.
230  littlefs is available in Mbed OS as the [LittleFileSystem] class.
231
232- [SPIFFS] - Another excellent embedded filesystem for NOR flash. As a more
233  traditional logging filesystem with full static wear-leveling, SPIFFS will
234  likely outperform littlefs on small memories such as the internal flash on
235  microcontrollers.
236
237- [Dhara] - An interesting NAND flash translation layer designed for small
238  MCUs. It offers static wear-leveling and power-resilience with only a fixed
239  _O(|address|)_ pointer structure stored on each block and in RAM.
240
241
242[BSD-3-Clause]: https://spdx.org/licenses/BSD-3-Clause.html
243[littlefs-fuse]: https://github.com/geky/littlefs-fuse
244[FUSE]: https://github.com/libfuse/libfuse
245[littlefs-js]: https://github.com/geky/littlefs-js
246[littlefs-js-demo]:http://littlefs.geky.net/demo.html
247[mklfs]: https://github.com/whitecatboard/Lua-RTOS-ESP32/tree/master/components/mklfs/src
248[Lua RTOS]: https://github.com/whitecatboard/Lua-RTOS-ESP32
249[Mbed OS]: https://github.com/armmbed/mbed-os
250[LittleFileSystem]: https://os.mbed.com/docs/mbed-os/v5.12/apis/littlefilesystem.html
251[SPIFFS]: https://github.com/pellepl/spiffs
252[Dhara]: https://github.com/dlbeer/dhara
253