4ad09d6c4e
This is the help the introduction of littlefs v2, which is disk incompatible with littlefs v1. While v2 can't mount v1, what we can do is provide an optional migration, which can convert v1 into v2 partially in-place. At worse, we only need to carry over the readonly operations on v1, which are much less complicated than the write operations, so the extra code cost may be as low as 25% of the v1 code size. Also, because v2 contains only metadata changes, it's possible to avoid copying file data during the update. Enabling the migration requires two steps 1. Defining LFS_MIGRATE 2. Call lfs_migrate (only available with the above macro) Each macro multiplies the number of configurations needed to be tested, so I've been avoiding macro controlled features since there's still work to be done around testing the single configuration that's already available. However, here the cost would be too high if we included migration code in the standard build. We can't use the lfs_migrate function for link time gc because of a dependency between the allocator and v1 data structures. So how does lfs_migrate work? It turned out to be a bit complicated, but the answer is a multistep process that relies on mounting v1 readonly and building the metadata skeleton needed by v2. 1. For each directory, create a v2 directory 2. Copy over v1 entries into v2 directory, including the soft-tail entry 3. Move head block of v2 directory into the unused metadata block in v1 directory. This results in both a v1 and v2 directory sharing the same metadata pair. 4. Finally, create a new superblock in the unused metadata block of the v1 superblock. Just like with normal metadata updates, the completion of the write to the second metadata block marks a succesful migration that can be mounted with littlefs v2. And all of this can occur atomically, enabling complete fallback if power is lost of an error occurs. Note there are several limitations with this solution. 1. While migration doesn't duplicate file data, it does temporarily duplicate all metadata. This can cause a device to run out of space if storage is tight and the filesystem as many files. If the device was created with >~2x the expected storage, it should be fine. 2. The current implementation is not able to recover if the metadata pairs develop bad blocks. It may be possilbe to workaround this, but it creates the problem that directories may change location during the migration. The other solutions I've looked at are complicated and require superlinear runtime. Currently I don't think it's worth fixing this limitation. 3. Enabling the migration requires additional code size. Currently this looks like it's roughly 11% at least on x86. And, if any failure does occur, no harm is done to the original v1 filesystem on disk.
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.
mklfs - A command line tool built by the Lua RTOS guys for making littlefs images from a host PC. Supports Windows, Mac OS, and Linux.
SPIFFS - Another excellent embedded filesystem for NOR flash. As a more traditional logging filesystem with full static wear-leveling, SPIFFS will likely outperform littlefs on small memories such as the internal flash on microcontrollers.
Dhara - An interesting NAND flash translation layer designed for small MCUs. It offers static wear-leveling and power-resilience with only a fixed O(|address|) pointer structure stored on each block and in RAM.