aeca7667b3
Instead of storing files in an arbitrary order, we now store files in ascending lexicographical order by filename. Although a big change, this actually has little impact on how littlefs works internally. We need to support file insertion, and compare file names to find our position. But since we already need to scan the entire directory block, this adds relatively little overhead. What this does allow, is the potential to add B-tree support in the future in a backwards compatible manner. How could you add B-trees to littlefs? 1. Add an optional "child" tag with a pointer that allows you to skip to a position in the metadata-pair list that composes the directory 2. When splitting a metadata-pair (sound familiar?), we either insert a second child tag in our parent, or we create a new root containing the child tags. 3. Each layer needs a bit stored in the tail-pointer to indicate if we're going to the next layer. This can be created trivially when we create a new root. 4. During lookup we keep two pointers containing the bounds of our search. We may need to iterate through multiple metadata-pairs in our linked-list, but this gives us a O(log n) lookup cost in a balanced tree. 5. During deletion we also delete any children pointers. Note that children pointers must come before the actual file entry. This gives us a B-tree implementation that is compatible with the current directory layout (assuming the files are ordered). This means that B-trees could be supported by a host PC and ignored on a small device. And during power-loss, we never end up with a broken filesystem, just a less-than-optimal tree. Note that we don't handle removes, so it's possible for a tree to become unbalanced. But worst case that's the same as the current linked-list implementation. All we need to do now is keep directories ordered. If we decide to drop B-tree support in the future or the B-tree implementation turns out inherently flawed, we can just drop the ordered requirement without breaking compatibility and recover the code cost.
The little filesystem
A little fail-safe filesystem designed for embedded systems.
| | | .---._____
.-----. | |
--|o |---| littlefs |
--| |---| |
'-----' '----------'
| | |
Bounded RAM/ROM - The littlefs is designed to work with a limited amount of memory. Recursion is avoided and dynamic memory is limited to configurable buffers that can be provided statically.
Power-loss resilient - The littlefs is designed for systems that may have random power failures. The littlefs has strong copy-on-write guarantees and storage on disk is always kept in a valid state.
Wear leveling - Since the most common form of embedded storage is erodible flash memories, littlefs provides a form of dynamic wear leveling for systems that can not fit a full flash translation layer.
Example
Here's a simple example that updates a file named boot_count every time
main runs. The program can be interrupted at any time without losing track
of how many times it has been booted and without corrupting the filesystem:
#include "lfs.h"
// variables used by the filesystem
lfs_t lfs;
lfs_file_t file;
// configuration of the filesystem is provided by this struct
const struct lfs_config cfg = {
// block device operations
.read = user_provided_block_device_read,
.prog = user_provided_block_device_prog,
.erase = user_provided_block_device_erase,
.sync = user_provided_block_device_sync,
// block device configuration
.read_size = 16,
.prog_size = 16,
.block_size = 4096,
.block_count = 128,
.cache_size = 16,
.lookahead_size = 16,
};
// entry point
int main(void) {
// mount the filesystem
int err = lfs_mount(&lfs, &cfg);
// reformat if we can't mount the filesystem
// this should only happen on the first boot
if (err) {
lfs_format(&lfs, &cfg);
lfs_mount(&lfs, &cfg);
}
// read current count
uint32_t boot_count = 0;
lfs_file_open(&lfs, &file, "boot_count", LFS_O_RDWR | LFS_O_CREAT);
lfs_file_read(&lfs, &file, &boot_count, sizeof(boot_count));
// update boot count
boot_count += 1;
lfs_file_rewind(&lfs, &file);
lfs_file_write(&lfs, &file, &boot_count, sizeof(boot_count));
// remember the storage is not updated until the file is closed successfully
lfs_file_close(&lfs, &file);
// release any resources we were using
lfs_unmount(&lfs);
// print the boot count
printf("boot_count: %d\n", boot_count);
}
Usage
Detailed documentation (or at least as much detail as is currently available) can be found in the comments in lfs.h.
As you may have noticed, littlefs takes in a configuration structure that defines how the filesystem operates. The configuration struct provides the filesystem with the block device operations and dimensions, tweakable parameters that tradeoff memory usage for performance, and optional static buffers if the user wants to avoid dynamic memory.
The state of the littlefs is stored in the lfs_t type which is left up
to the user to allocate, allowing multiple filesystems to be in use
simultaneously. With the lfs_t and configuration struct, a user can
format a block device or mount the filesystem.
Once mounted, the littlefs provides a full set of POSIX-like file and directory functions, with the deviation that the allocation of filesystem structures must be provided by the user.
All POSIX operations, such as remove and rename, are atomic, even in event of power-loss. Additionally, no file updates are actually committed to the filesystem until sync or close is called on the file.
Other notes
All littlefs have the potential to return a negative error code. The errors
can be either one of those found in the enum lfs_error in lfs.h,
or an error returned by the user's block device operations.
In the configuration struct, the prog and erase function provided by the
user may return a LFS_ERR_CORRUPT error if the implementation already can
detect corrupt blocks. However, the wear leveling does not depend on the return
code of these functions, instead all data is read back and checked for
integrity.
If your storage caches writes, make sure that the provided sync function
flushes all the data to memory and ensures that the next read fetches the data
from memory, otherwise data integrity can not be guaranteed. If the write
function does not perform caching, and therefore each read or write call
hits the memory, the sync function can simply return 0.
Reference material
DESIGN.md - DESIGN.md contains a fully detailed dive into how littlefs actually works. I would encourage you to read it since the solutions and tradeoffs at work here are quite interesting.
SPEC.md - SPEC.md contains the on-disk specification of littlefs with all the nitty-gritty details. Can be useful for developing tooling.
Testing
The littlefs comes with a test suite designed to run on a PC using the emulated block device found in the emubd directory. The tests assume a Linux environment and can be started with make:
make test
License
The littlefs is provided under the BSD-3-Clause license. See LICENSE.md for more information. Contributions to this project are accepted under the same license.
Individual files contain the following tag instead of the full license text.
SPDX-License-Identifier: BSD-3-Clause
This enables machine processing of license information based on the SPDX License Identifiers that are here available: http://spdx.org/licenses/
Related projects
Mbed OS - The easiest way to get started with littlefs is to jump into Mbed, which already has block device drivers for most forms of embedded storage. The littlefs is available in Mbed OS as the LittleFileSystem class.
littlefs-fuse - A FUSE wrapper for littlefs. The project allows you to mount littlefs directly on a Linux machine. Can be useful for debugging littlefs if you have an SD card handy.
littlefs-js - A javascript wrapper for littlefs. I'm not sure why you would want this, but it is handy for demos. You can see it in action here.