Christopher Haster
42ec282a03
One downside of leb128 encoding is that the worst case encoded size is
not that well aligned due to a relatively underutilized last byte:
0xffffffff => 0xff 0xff 0xff 0xff 0x0f
This normally doesn't really matter, the whole point of leb128 is that
larger encodings are statistically less likely. But in littlefs we need
to allocate the worst-case buffer size in order to encode/decode
leb128s, and these buffers need to stick around on the stack during
metadata commit calls, which are also the point of highest stack usage
in the system.
But 32-bits is somewhat arbitrary, it just happens to be our register
size. In fact, we're not really using 32-bits, but instead only 31-bits
to take advantage of the sign bit for ad-hoc sum types:
0x7fffffff => 0xff 0xff 0xff 0xff 0x07
In theory, if we limit this further to 28-bits, we could save some stack
space:
0x7fffffff => 0xff 0xff 0xff 0xff 0x07
0x0fffffff => 0xff 0xff 0xff 0x7f
This may seem like a small amount of savings, but it also restores
alignment to the encoding, and should result in less wasted padding
around buffers.
Though it's important to note these are the most valuable bits, as the
range grows exponentially with each bit added. Reducing 31-bits to
28-bits reduces the range from ~2GiB to ~256MiB:
0x7fffffff => 2,147,483,647
0x0fffffff => 268,435,455
---
At the moment I'm hesistant to reduce _all_ on-disk leb128s to 28-bits.
The signed-32-bit limit of ~2GiB is fairly well understood in this
space, mainly thanks to FAT, and reducing this to ~256MiB risks quite a
surprise to users (it's also a regression from the current littlefs
version).
But one type where this limit is pretty reasonable is our block_size.
I don't think we'll see devices with erase blocks >256MiB for a while,
and at the very least those devices will probably need a 64-bit
filesystem for other reasons anyways...
And limiting block_size to <=256MiB has a surprising number of knock-on
effects:
- The tag size/jump field never exceeds 28-bits, reducing worst-case tag
dsize from 12 bytes -> 11 bytes.
The also reduces our worst-case attr-estimate from 40 bytes ->
37 bytes
- rbyd/btree trunks never exceed 28-bits, saving space in shrub/branch/
btree encodings.
- The bptr encoding is reduced from 24 bytes -> 21 bytes, since several
of its fields are in-block (size, off, cksize).
- The commit checksum encoding is reduced by a byte for every commit,
from 12 bytes -> 11 bytes.
This is due to needing to expand the cksum tag's size field to the
worst possible leb128 encoding due to a catch-22 situation.
Unfortunately the actual stack savings is a bit underwhelming:
code stack
before: 33688 2808
after: 33700 (+0.0%) 2800 (-0.3%)
This may be because, by adopting 28-bits in only some fields, most
buffers still end up unaligned and the on-stack size doesn't change due
to padding. Or it could just be that I'm overestimating the cost of our
on-stack buffers.
Still, I think the change is worth keeping if only for the reducing
attr-estimate and saved byte on every on-disk commit.
In the future it would be interesting to explore additional
configurations, e.g. a 28-bit flavor of littlefs to compliment this
31-bit flavor. You could imagine the fitting into other register sized
flavors for different capacity/code cost/device compat tradeoffs:
flavor register leb128 size-limit
14-bit littlefs => 16-bit 2 bytes ~16KiB
15-bit littlefs => 16-bit 3 bytes ~32KiB
28-bit littlefs => 32-bit 4 bytes ~256MiB
31-bit littlefs => 32-bit 5 bytes ~2GiB
56-bit littlefs => 64-bit 8 bytes ~64PiB
63-bit littlefs => 64-bit 9 bytes ~8ExiB
This is where the on-disk size-limit attr would really shine.
---
Note we don't need an additional on-disk limit attr for the block_size.
We already store the block_size in the superblock, so we just need to
error if attempting to mount a filesystem with block_size >256MiB.
littlefs
A little fail-safe filesystem designed for microcontrollers.
| | | .---._____
.-----. | |
--|o |---| littlefs |
--| |---| |
'-----' '----------'
| | |
Power-loss resilience - littlefs is designed to handle random power failures. All file operations have strong copy-on-write guarantees and if power is lost the filesystem will fall back to the last known good state.
Dynamic wear leveling - littlefs is designed with flash in mind, and provides wear leveling over dynamic blocks. Additionally, littlefs can detect bad blocks and work around them.
Bounded RAM/ROM - littlefs is designed to work with a small amount of memory. RAM usage is strictly bounded, which means RAM consumption does not change as the filesystem grows. The filesystem contains no unbounded recursion and dynamic memory is limited to configurable buffers that can be provided statically.
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,
.block_cycles = 500,
};
// 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.
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, file updates are not actually committed to the filesystem until sync or close is called on the file.
Other notes
Littlefs is written in C, and specifically should compile with any compiler
that conforms to the C99 standard.
All littlefs calls 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.
Design
At a high level, littlefs is a block based filesystem that uses small logs to store metadata and larger copy-on-write (COW) structures to store file data.
In littlefs, these ingredients form a sort of two-layered cake, with the small logs (called metadata pairs) providing fast updates to metadata anywhere on storage, while the COW structures store file data compactly and without any wear amplification cost.
Both of these data structures are built out of blocks, which are fed by a common block allocator. By limiting the number of erases allowed on a block per allocation, the allocator provides dynamic wear leveling over the entire filesystem.
root
.--------.--------.
| A'| B'| |
| | |-> |
| | | |
'--------'--------'
.----' '--------------.
A v B v
.--------.--------. .--------.--------.
| C'| D'| | | E'|new| |
| | |-> | | | E'|-> |
| | | | | | | |
'--------'--------' '--------'--------'
.-' '--. | '------------------.
v v .-' v
.--------. .--------. v .--------.
| C | | D | .--------. write | new E |
| | | | | E | ==> | |
| | | | | | | |
'--------' '--------' | | '--------'
'--------' .-' |
.-' '-. .-------------|------'
v v v v
.--------. .--------. .--------.
| F | | G | | new F |
| | | | | |
| | | | | |
'--------' '--------' '--------'
More details on how littlefs works can be found in DESIGN.md and SPEC.md.
-
DESIGN.md - A fully detailed dive into how littlefs works. I would suggest reading it as the tradeoffs at work are quite interesting.
-
SPEC.md - The on-disk specification of littlefs with all the nitty-gritty details. May be useful for tooling development.
Testing
The littlefs comes with a test suite designed to run on a PC using the
emulated block device found in the bd 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
-
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.
-
littlefs-python - A Python wrapper for littlefs. The project allows you to create images of the filesystem on your PC. Check if littlefs will fit your needs, create images for a later download to the target memory or inspect the content of a binary image of the target memory.
-
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.
-
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. littlefs is available in Mbed OS as the LittleFileSystem class.
-
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.