Christopher Haster
d61c7ca407
The motivation for this is that the rbyd inner node encoding during
compaction is kind-of not that great.
Our alt encoding is great when the trunk terminates in a tag, which is
how it was originally designed to be used:
00000004: data w1 1 61 a <.
00000020: altble 0x300 w1 0x4 -' <-- trunk
00000024: data w1 1 62 b
But when used to create an arbitrary binary-tree inner node, the best
encoding I can think of is 2 alts + a terminating null tag, which is not
that great:
00000004: data w1 1 61 a <.
00000009: data w1 1 62 b <--.
0000000e: altble 0x300 w1 0x4 -' | <-- trunk
00000012: altble 0x300 w1 0x9 ---'
00000016: null
This effects our attr estimate, which is defined as the worst-case
on-disk cost of an attr after compaction. This is an important value,
as it determines when we split rbyds. And it effectively determines how
densely we can store metadata without needing to worry about block
overflow issues.
In our current compaction algorithm, we connect each attr with a 2 alt +
null inner node. Since we are creating a perfectly balanced binary tree,
this works out to ~1 inner node per attr. Including the attr's data tag,
this gives us:
a_0 = 3t + 4
Where t is the tag estimate, currently a 2 byte tag, <=5 byte weight,
<=4 byte size, t = 2+5+4 = 11 bytes:
a_0 = 3*11 + 4 = 37 bytes
Though this may be vary across different littlefs configurations,
16-bit, 64-bit, etc.
---
It would be great if our compaction algorithm could build each trunk
perfectly, as each attr is written. In such a case, each attr
theoretically only needs ~1 alt and the attr's data tag:
a_inf = 2t
Or, assuming t = 11 bytes:
a_inf = 2*11 = 22 bytes
Unfortunately, as far as I can tell, this fundamentally requires
unbounded RAM. You need to keep track of log n previous trunks in order
to always build the next trunk perfectly, and log n is > 1.
I suppose in theory you could implement a O(n^2) algorithm that
repeatedly scans for the previous trunks... But that would be a
hilarious regression since the whole point of this work was to reduce
compaction from O(n^2) -> O(log n).
---
However, we can meet halfway. Consider what happens if we build perfect
trunks for only the bottom layer of the rbyd.
This may not seem like it will gain much, but remember that in a binary
tree, the bottom layer contains ~1/2 of the total nodes in the tree:
3t + 4 2t 5t
a_1 = ------ + -- = -- + 2
2 2 2
Or, assuming t = 11 bytes:
5*11
a_1 = ---- + 2 = 30 bytes
2
Not too shabby for a constant amount of RAM.
In theory this could be extended to n layers, by keeping a
(configurable?) array of previous trunks in RAM during compaction, but
this would have diminishing results as we move up the tree.
With only needing to keep track of one other trunk, we can even store
this in rybd.trunk, which is currently unused during compaction. So zero
extra RAM.
The resulting compaction looks like the following:
00000004: data w1 1 61 a <.
00000020: altble 0x300 w1 0x4 <--.
00000024: data w1 1 62 b |
00000029: data w1 1 63 c <. |
0000002e: altble 0x300 w1 0x29 <. |
00000032: data w1 1 64 d | |
00000037: altble 0x300 w2 0x20 -' | <-- trunk
0000003b: altble 0x300 w2 0x2e ---'
0000003f: null
This does make compaction a bit more complicated, which is reflected in
the code size:
code stack
before: 33856 2880
after: 34068 (+0.6%) 2880 (+0.0%)
Some implementation things to note:
- Since lfsr_rbyd_appendcompactattr now has state, we need to make sure
to zero the trunk before compacting, which adds an annoying bit of
bookkeeping everywhere.
- I didn't want to add extra state to manage in
lfsr_rbyd_appendcompactattr calls, so this implementation only tracks
the previous trunk offset, and needs a readtag call to get the
tag+weight necessary to build the actual alt pointer.
This may have some unpleasant interactions with the rcache, and may be
worth revisiting.
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.