a49e13b992
The idea here, is we give each lfsr_btree_t an optional leaf rbyd, in
addition to the root rbyd. This leaf rbyd acts as a cache for the most
recent leaf, allowing nearby btree lookups to skip the full btree walk.
Unfortunately, this failed on pretty much every measurable metric...
---
The motivation for this is that we often do a bunch of nearby btree
lookups:
- Btree iteration via lfsr_btree_lookupnext is a bit naive, walking from
the root every step.
- Our crystallization algorithm requires a bunch of nearby lookups to
figure out our crystallization heuristic. Currently at most 4, when
you need to lookup both crystal neighbors and then _also_ both
fragment neighbors for coalescing.
- Checksum collision resolution for dids and (FUTURE) ddkeys can require
an unbounded number of sequential lookups.
Though to be fair, this is an exceptional case if our checksum is any
good.
- Bids with multiple rattrs require nearby lookups to resolve.
Though currently this can be explicitly avoided via
lfsr_btree_lookupleaf + lfsr_rbyd_lookup.
The theory was that cases like these could explicitly keep track of the
leaf rbyd to avoid full btree walks, but in practice this never really
worked out. Tracking if we're still in the relevant leaf rbyd just adds
too much logic/code cost.
But if this leaf tracking logic was implemented once in the btree
layer...
The other theoretical benefit was being able to move more rbyds off the
stack. Sure our btrees take up more RAM, but if that results in stack
savings, that may be a win.
Oh, and this would let our btree API and rbyd API converge without
performance concerns. Internal users could in theory call
lfsr_btree_lookupnext + lfsr_btree_lookup with the same performance as
explicitly tracking the rbyd.
---
But this was a complete failure!
First the good news: There was a modest speedup of around ~2x to linear
reads.
And that's the good news.
Now the bad news:
1. There was no noticeable performance gain in any other benchmarks.
To be fair, we're at the early stages of benchmarking, so the
benchmarks may not be the most thorough, but thinking about it, there
are some explanations:
- In any benchmark that writes, fetch + erase + prog dominates. Being
able to skip fetches during lookups makes our btree lookups
surprisingly cheap!
- Any random read heavy benchmark is likely thrashing this cache,
which is to be expected.
- For small 1-block btrees, the leaf cache is useless because the
entire btree is cache in the root rbyd.
And keep in mind, our blocks are BIG. "Small" here could be on
the order of ~128KiB-1MiB for NAND flash.
- For the mtree, fetched mdirs actually already act as a sort of leaf
cache.
The extra btree leaf cache isn't doing _nothing_, but each layer of
the mtree has diminishing returns due to btree's ridiculous
branching factor.
- For file btrees, we're explicitly caching the leaf fragments/
blocks, so the extra btree leaf cache has diminishing returns for
the same reason.
2. Code cost was bad, stack cost was worse:
code stack ctx
before: 37172 2288 636
after: 38068 (+2.4%) 2416 (+5.6%) 664 (+4.4%)
Tracking the leaf required more code, that's expected. And, to be
fair, the current code has had a lot more time to congeal.
What wasn't expected was the stack cost.
Unfortunately these caches didn't really take any rbyds off the stack
hot-path:
- We _can_ get rid of the rbyd in lfsr_btree_lookup/namelookup, but
we were already hacking our way around the critical one in
lfsr_mtree_lookup/namelookup by reusing the mdir's rbyd!
- We can't even abuse the leaf rbyd in the commit logic, since the
target btree can end up iterated/traversed by lfs_alloc.
That was a fun bug.
And the addition of a second rbyd to lfsr_btree_t increases both ctx
and stack anywhere btrees are allocated.
Maybe this will make more sense when we add the auxiliary btrees, or
after more benchmarking, but for now the theoretical performance
improvements just aren't worth it.
Will probably revert this, but I wanted to commit it in case the idea is
worth resurrecting in the future, if in the future nearby btree lookups
are a bigger penalty than they are now.
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.