开源鸿蒙 LittleFS

2022-08-09
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LittleFS

Basic Concepts

LittleFS is a small file system designed for flash. By combining the log-structured file system and the copy-on-write (COW) file system, LittleFS stores metadata in log structure and data in the COW structure. This special storage empowers LittleFS high power-loss resilience. LittleFS uses the statistical wear leveling algorithm when allocating COW data blocks, effectively prolonging the service life of flash devices. LittleFS is designed for small-sized devices with limited resources, such as ROM and RAM. All RAM resources are allocated through a buffer with the fixed size (configurable). That is, the RAM usage does not grow with the file system.

LittleFS is a good choice when you look for a flash file system that is power-cut resilient and has wear leveling support on a small device with limited resources.

Development Guidelines

When porting LittleFS to a new hardware device, you need to declare lfs_config:

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,
};

.read, .prog, .erase, and .sync correspond to the read, write, erase, and synchronization APIs at the bottom layer of the hardware platform, respectively.

read_size indicates the number of bytes read each time. You can set it to a value greater than the physical read unit to improve performance. This value determines the size of the read cache. However, if the value is too large, more memory is consumed.

prog_size indicates the number of bytes written each time. You can set it to a value greater than the physical write unit to improve performance. This value determines the size of the write cache and must be an integral multiple of read_size. However, if the value is too large, more memory is consumed.

block_size: indicates the number of bytes in each erase block. The value can be greater than that of the physical erase unit. However, a smaller value is recommended because each file occupies at least one block. The value must be an integral multiple of prog_size.

block_count indicates the number of blocks that can be erased, which depends on the capacity of the block device and the size of the block to be erased (block_size).

Sample Code

The sample code is as follows:

#include "lfs.h"
#include "stdio.h"
lfs_t lfs;
lfs_file_t file;
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,
};
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);
}

Verification

The development is successful if the return result is as follows: