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
/
.
Block devices¶
A block device is an instance of a class that implements the
vfs.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¶
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 vfs
bdev = RAMBlockDev(512, 50)
vfs.VfsFat.mkfs(bdev)
vfs.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
vfs.AbstractBlockDev.readblocks()
and
vfs.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 vfs
bdev = RAMBlockDev(512, 50)
vfs.VfsLfs2.mkfs(bdev)
vfs.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 (1M) |
No |
No |
Yes |
ESP8266 (2M+) |
Yes |
No |
Yes |
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 vfs
vfs.umount('/')
vfs.VfsFat.mkfs(bdev)
vfs.mount(bdev, '/')
# STM32
import os, vfs, pyb
vfs.umount('/flash')
vfs.VfsFat.mkfs(pyb.Flash(start=0))
vfs.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.
To format the entire flash using littlefs v2:
# ESP8266 and ESP32
import vfs
vfs.umount('/')
vfs.VfsLfs2.mkfs(bdev)
vfs.mount(bdev, '/')
# STM32
import os, vfs, pyb
vfs.umount('/flash')
vfs.VfsLfs2.mkfs(pyb.Flash(start=0))
vfs.mount(pyb.Flash(start=0), '/flash')
os.chdir('/flash')
A littlefs filesystem can be still be accessed on a PC over USB MSC using the
littlefs FUSE driver. Note that you must specify both the --block_size
and --block_count
options to override the defaults. For example (after
building the littlefs-fuse executable):
$ ./lfs --block_size=4096 --block_count=512 -o allow_other /dev/sdb1 mnt
This will allow the board’s littlefs filesystem to be accessed at the mnt
directory. To get the correct values of block_size
and block_count
use:
import pyb
f = pyb.Flash(start=0)
f.ioctl(1, 1) # initialise flash in littlefs raw-block mode
block_count = f.ioctl(4, 0)
block_size = f.ioctl(5, 0)
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, vfs, pyb
vfs.umount('/flash')
p1 = pyb.Flash(start=0, len=256*1024)
p2 = pyb.Flash(start=256*1024)
vfs.VfsFat.mkfs(p1)
vfs.VfsLfs2.mkfs(p2)
vfs.mount(p1, '/flash')
vfs.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 vfs, pyb
p2 = pyb.Flash(start=256*1024)
vfs.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, vfs
p = esp32.Partition.find(esp32.Partition.TYPE_DATA, label='foo')
vfs.mount(p, '/foo')