Christopher Haster
c370fbec1a
The issue, found in test_files_many:h1g4j10l18, occurs when a SUBWIDE
tag follows a compaction.
When this happens, it's possible for our stitched diverging alt to be
followed/flipped when it shouldn't be. This is because the new
lower_rid/upper_rid window can make the stitched alt ambiguous.
I don't think this is strictly an issue with compaction, as much as
compaction is giving us a tree structure that's not reachable through
only appendattrs.
Here are the three culprit trunks:
altrle 0x300 w8 0x2c8
altble 0x203 w6 0x2d4 <- diverge
null
altrle 0x300 w8 0x2c8
altbgt 0x203 w0 0x2e8 <- diverge
altble 0x300 w4 0x2b4
altbn w0 0x0
altble 0x300 w1 0x228
altbn w0 0x0
null
altrle 0x300 w8 0x2c8
altbgt 0x300 w0 0x2e8 <- stitch
altbn w0 0x0
altbn w0 0x0
altbn w0 0x0
altbgt 0x201 w0 0x164
reg w1
And here is a simplified view, after compaction, before we do a subwide
append/replace:
.-> reg w1 a
.-------b-> data
| .-> reg w1 b
| .---b-> data
r-b-r-b-b-> orphan w1 <- removed as a part of our subwide op
^
diverging
'-+-'
weight=3
altrle data w1
altble orphan w1
First, as a part of our subwide append, we're going to write out the
lower trunk. We diverge on the first altble since the entire orphan is
inside our subwide range.
It may seem a bit strange to diverge on a null tag, but this isn't
actually an issue, we're allowed a single null tag to terminate our
tree:
.-> reg w1 a
.-------b-> data
| .-> reg w1 b
r-b-----b-> data
^
diverging
'-+-'
weight=3
altrle data w1
altbgt data w0
Nothing wrong so far. The weight of our leaves (2) don't match our
tree's weight (3), but this is normal for the lower trunk. We fix this
when we stitch the diverging alt on the upper trunk.
Speaking of the upper trunk, let's start writing it out, but pause at
the stitching alt:
.-> reg w1 a
.-------b-> data
| .-> reg w1 b
| .-----b-> data
r-b-?
^
stitching
'-+-'
weight=3
altrle data w1
altble data w1
Note we've flipped the altbgt data into an altble data, since we're
going down the other diverged path now.
But before we continue, as a part of stitching, we need to adjust our
tree weight to account for the weight of the orphan we deleted as a part
of our range operation:
.-> reg w1 a
.-------b-> data
| .-> reg w1 b
| .-----b-> data
r-b-?
^
stitching
'-+-'
weight=2
altrle data w1
altble data w1
Uh oh. Weight is 2 and both our alts add up to 2? All of a sudden it
looks like we should follow the stitched alt.
Our follow/flip logic kicks in, and disaster!
.-> reg w1 a
.-------b-> data
r-b-----b-> reg w1 c <- added as a part of our subwide op
'-> data <- somehow data survives
'-+-' but where did b go?
weight=2
altrle data w1
altbgt data w0
We go down the wrong path, and because our state machine thinks we've
diverged, we prune all le alts, destroying our tree.
---
So what's is going wrong?
The problem is that when we update our window, the stitched diverging
alt can become ambiguous.
Which sort of makes sense. The reason we update our window is so we can
continue down the tree veiwing it as it was _before_ the range
operation. But the stitched alt belongs to the tree _after_ the range
operation.
The solution here is to just make sure we never follow the stitched alt.
This is a bit annoying, as it makes the stitched alt a rather special
case, but as far as I can tell it's necessary to avoid ambiguity.
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.