Working with filesystems

This tutorial describes how MicroPython provides an on-device filesystem, allowing standard Python file I/O methods to be used with persistent storage.

MicroPython automatically creates a default configuration and auto-detects the primary filesystem, so this tutorial will be mostly useful if you want to modify the partitioning, filesystem type, or use custom block devices.

The filesystem is typically backed by internal flash memory on the device, but can also use external flash, RAM, or a custom block device.

On some ports (e.g. STM32), the filesystem may also be available over USB MSC to a host PC. The pyboard.py tool also provides a way for the host PC to access to the filesystem on all ports.

Note: This is mainly for use on bare-metal ports like STM32 and ESP32. On ports with an operating system (e.g. the Unix port) the filesystem is provided by the host OS.

VFS

MicroPython implements a Unix-like Virtual File System (VFS) layer. All mounted filesystems are combined into a single virtual filesystem, starting at the root /. Filesystems are mounted into directories in this structure, and at startup the working directory is changed to where the primary filesystem is mounted.

On STM32 / Pyboard, the internal flash is mounted at /flash, and optionally the SDCard at /sd. On ESP8266/ESP32, the primary filesystem is mounted at /. On the OpenMV Cam the internal flash is mounted at / unless an SDCard is installed which will be moutned at / instead.

Block devices

A block device is an instance of a class that implements the uos.AbstractBlockDev protocol.

Built-in block devices

Ports provide built-in block devices to access their primary flash.

On power-on, MicroPython will attempt to detect the filesystem on the default flash and configure and mount it automatically. If no filesystem is found, MicroPython will attempt to create a FAT filesystem spanning the entire flash. Ports can also provide a mechanism to “factory reset” the primary flash, usually by some combination of button presses at power on.

STM32 / Pyboard / OpenMV Cam

The pyb.Flash class provides access to the internal flash. On some boards which have larger external flash (e.g. Pyboard D), it will use that instead. The start kwarg should always be specified, i.e. pyb.Flash(start=0).

Note: For backwards compatibility, when constructed with no arguments (i.e. pyb.Flash()), it only implements the simple block interface and reflects the virtual device presented to USB MSC (i.e. it includes a virtual partition table at the start).

ESP8266

The internal flash is exposed as a block device object which is created in the flashbdev module on start up. This object is by default added as a global variable so it can usually be accessed simply as bdev. This implements the extended interface.

ESP32

The esp32.Partition class implements a block device for partitions defined for the board. Like ESP8266, there is a global variable bdev which points to the default partition. This implements the extended interface.

Custom block devices

The following class implements a simple block device that stores its data in RAM using a bytearray:

class RAMBlockDev:
    def __init__(self, block_size, num_blocks):
        self.block_size = block_size
        self.data = bytearray(block_size * num_blocks)

    def readblocks(self, block_num, buf):
        for i in range(len(buf)):
            buf[i] = self.data[block_num * self.block_size + i]

    def writeblocks(self, block_num, buf):
        for i in range(len(buf)):
            self.data[block_num * self.block_size + i] = buf[i]

    def ioctl(self, op, arg):
        if op == 4: # get number of blocks
            return len(self.data) // self.block_size
        if op == 5: # get block size
            return self.block_size

It can be used as follows:

import os

bdev = RAMBlockDev(512, 50)
os.VfsFat.mkfs(bdev)
os.mount(bdev, '/ramdisk')

An example of a block device that supports both the simple and extended interface (i.e. both signatures and behaviours of the uos.AbstractBlockDev.readblocks() and uos.AbstractBlockDev.writeblocks() methods) is:

class RAMBlockDev:
    def __init__(self, block_size, num_blocks):
        self.block_size = block_size
        self.data = bytearray(block_size * num_blocks)

    def readblocks(self, block_num, buf, offset=0):
        addr = block_num * self.block_size + offset
        for i in range(len(buf)):
            buf[i] = self.data[addr + i]

    def writeblocks(self, block_num, buf, offset=None):
        if offset is None:
            # do erase, then write
            for i in range(len(buf) // self.block_size):
                self.ioctl(6, block_num + i)
            offset = 0
        addr = block_num * self.block_size + offset
        for i in range(len(buf)):
            self.data[addr + i] = buf[i]

    def ioctl(self, op, arg):
        if op == 4: # block count
            return len(self.data) // self.block_size
        if op == 5: # block size
            return self.block_size
        if op == 6: # block erase
            return 0

As it supports the extended interface, it can be used with littlefs:

import os

bdev = RAMBlockDev(512, 50)
os.VfsLfs2.mkfs(bdev)
os.mount(bdev, '/ramdisk')

Once mounted, the filesystem (regardless of its type) can be used as it normally would be used from Python code, for example:

with open('/ramdisk/hello.txt', 'w') as f:
    f.write('Hello world')
print(open('/ramdisk/hello.txt').read())

Filesystems

MicroPython ports can provide implementations of FAT, littlefs v1 and littlefs v2.

The following table shows which filesystems are included in the firmware by default for given port/board combinations, however they can be optionally enabled in a custom firmware build.

Board FAT littlefs v1 littlefs v2
pyboard 1.0, 1.1, D Yes No Yes
Other STM32 Yes No No
ESP8266 Yes No No
ESP32 Yes No Yes

FAT

The main advantage of the FAT filesystem is that it can be accessed over USB MSC on supported boards (e.g. STM32) without any additional drivers required on the host PC.

However, FAT is not tolerant to power failure during writes and this can lead to filesystem corruption. For applications that do not require USB MSC, it is recommended to use littlefs instead.

To format the entire flash using FAT:

# ESP8266 and ESP32
import os
os.umount('/')
os.VfsFat.mkfs(bdev)
os.mount(bdev, '/')

# STM32
import os, pyb
os.umount('/flash')
os.VfsFat.mkfs(pyb.Flash(start=0))
os.mount(pyb.Flash(start=0), '/flash')
os.chdir('/flash')

Littlefs

Littlefs is a filesystem designed for flash-based devices, and is much more resistant to filesystem corruption.

Note

There are reports of littlefs v1 and v2 failing in certain situations, for details see littlefs issue 347 and littlefs issue 295.

Note: It can be still be accessed over USB MSC using the littlefs FUSE driver. Note that you must use the -b=4096 option to override the block size.

To format the entire flash using littlefs v2:

# ESP8266 and ESP32
import os
os.umount('/')
os.VfsLfs2.mkfs(bdev)
os.mount(bdev, '/')

# STM32
import os, pyb
os.umount('/flash')
os.VfsLfs2.mkfs(pyb.Flash(start=0))
os.mount(pyb.Flash(start=0), '/flash')
os.chdir('/flash')

Hybrid (STM32)

By using the start and len kwargs to pyb.Flash, you can create block devices spanning a subset of the flash device.

For example, to configure the first 256kiB as FAT (and available over USB MSC), and the remainder as littlefs:

import os, pyb
os.umount('/flash')
p1 = pyb.Flash(start=0, len=256*1024)
p2 = pyb.Flash(start=256*1024)
os.VfsFat.mkfs(p1)
os.VfsLfs2.mkfs(p2)
os.mount(p1, '/flash')
os.mount(p2, '/data')
os.chdir('/flash')

This might be useful to make your Python files, configuration and other rarely-modified content available over USB MSC, but allowing for frequently changing application data to reside on littlefs with better resilience to power failure, etc.

The partition at offset 0 will be mounted automatically (and the filesystem type automatically detected), but you can add:

import os, pyb
p2 = pyb.Flash(start=256*1024)
os.mount(p2, '/data')

to boot.py to mount the data partition.

Hybrid (ESP32)

On ESP32, if you build custom firmware, you can modify partitions.csv to define an arbitrary partition layout.

At boot, the partition named “vfs” will be mounted at / by default, but any additional partitions can be mounted in your boot.py using:

import esp32, os
p = esp32.Partition.find(esp32.Partition.TYPE_DATA, label='foo')
os.mount(p, '/foo')