This guide will show you how to perform an over-the-air (OTA) root filesystem (rootfs) update with A/B partition support (fail safe) on a Raspberry Pi 4B device using Thistle. This guide uses an official Raspberry Pi OS image released by the Raspberry Pi Foundation. You can follow along with our video guide as well.
Thistle Control Center

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Tools Needed for This Tutorial

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Hardware

  • One Raspberry Pi 4 Model B single-board computer with accessories including a microSD card that’s 32GB or larger. Any CanaKit Raspberry Pi 4 Starter Kit will work. This is the target host device for OTA A/B update integration.
  • One USB-A thumb drive (64GB or larger), for example SanDisk 64GB Ultra USB 3.0 Flash Drive - SDCZ48-064G-UAM46. We will program the USB drive, and use it to boot the Raspberry Pi into a “utility” OS to prepare the microSD card for OTA A/B update integration.
  • One desktop/laptop PC running Linux, MacOS X, or Windows, for publishing OTA updates, programming the USB drive, controlling the Raspberry Pi, and observing its logs.
  • One USB to TTL Serial Cable - Debug / Console Cable for Raspberry Pi. If your PC runs Windows or MacOS X, you need to install the associated PL2303 and CP2102 drivers by following instructions in the preceding link. The cable is used to connect the PC to the Raspberry Pi’s UART port, to observe boot logs.

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Software

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OTA A/B Update Demo Preparation

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Connect PC and RPi-4 over Serial Port

Connect the RPi-4’s Pin 6 (GND), Pin 8 (GPIO 14 / TXD), and Pin 10 (GPIO 15 / RXD) to the Ground (black), RXD (white), and TXD (green) of the USB To TTL serial cable, respectively. Plug the other end of the cable (USB-A male) to the PC. Connect PC to RPi-4

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Prepare USB Drive and microSD Card on PC

On the desktop/laptop PC, install the Raspberry Pi Imager application, and use it to install a utility operating system to the USB drive. Insert the USB drive to the PC. In Imager’s UI, select “Raspberry Pi 4” as the device, “Raspberry Pi OS (other) > Raspberry Pi OS Lite (64-bit)” as the operating system (for this guide we use the May 13, 2025 version), and the USB drive as the storage. In the “OS customisation” step: set the host name (in this guide our host name will be “rpi4-util”), username and password; configure wireless LAN credential so that the RPi-4 can automatically connect to WiFi; and enable SSH to allow a headless connection setup. Program the USB drive with the selected OS with the custom setting. Remove the USB drive from PC. Insert the microSD card to the PC. Repeat the above process to write the Raspberry Pi OS with similar settings to the microSD card. We will use hostname “ota-demo” in this guide. Remove the microSD card from PC. The microSD card will be used for the OTA A/B update demonstration. Once both the USB drive and the microSD card are programmed, insert the USB drive to the RPi-4’s USB-A port. Make sure the microSD card is not inserted into the RPi-4. RPi-4 with USB Drive

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Boot into Utility OS and Install Necessary Software

Power on the Raspberry Pi 4 device, and wait a couple of minutes for the OS to boot up. Find the IP address of the device using a network scanning tool such as Nmap. SSH onto the RPi-4: ssh <username>@<rpi4-ip-addr>. Commands in the rest of this section are executed in the SSH shell on the RPi-4.
  1. Keep the Raspberry Pi OS and packages up to date, and get the latest EEPROM firmware. 
    thistle@rpi4-util:~ $ sudo apt update
thistle@rpi4-util:~ $ sudo apt dist-upgrade -y
thistle@rpi4-util:~ $ sudo rpi-eeprom-update -a
    Because of issues on the earlier versions of the Raspberry Pi 4, one needs to run on the latest EEPROM firmware.
  2. Run sudo raspi-config to enable the Serial Port interfaces. The option is under “Interface Options” in the main menu. Click on “Finish” to save the configuration changes, and reboot the RPi-4 for the changes to take effect. Run raspi-config
  3. Insert the microSD card after the utility OS has booted. We will customize the stock Raspberry Pi OS image to make it ready for the A/B update demo. On the Raspberry Pi in the utility OS, the microSD card should show up as /dev/mmcblk0. It contains two partitions: a boot partition mmcblk0p1, and a rootfs partition mmcblk0p2. 
    thistle@rpi4-util:~ $ lsblk /dev/mmcblk0
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
mmcblk0     179:0    0 58.9G  0 disk
├─mmcblk0p1 179:1    0  512M  0 part
└─mmcblk0p2 179:2    0  2.1G  0 part
    For A/B update to work, we need a third partition for the alternative rootfs, and will create it using fdisk on the utility OS interactively. 
    thistle@rpi4-util:~ $ sudo fdisk /dev/mmcblk0
    In the fdisk menu, first show the current partition table. Take note on the end sector number for /dev/mmcblk0p2 - we will create a new partition right after it. 
    Welcome to fdisk (util-linux 2.38.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Command (m for help): p
Disk /dev/mmcblk0: 58.94 GiB, 63281561600 bytes, 123596800 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xd9c86127

Device         Boot   Start     End Sectors  Size Id Type
/dev/mmcblk0p1        16384 1064959 1048576  512M  c W95 FAT32 (LBA)
/dev/mmcblk0p2      1064960 5390335 4325376  2.1G 83 Linux
    Now create a new primary partition /dev/mmcblk0p3, as follows. Set the first sector value the last sector number of /dev/mmcblk0p2 plus 1 (5390336 in this example), and use the default value for all other choices. 
    Command (m for help): n
Partition type
  p   primary (2 primary, 0 extended, 2 free)
  e   extended (container for logical partitions)
Select (default p): p
Partition number (3,4, default 3): 3
First sector (2048-123596799, default 2048): 5390336
Last sector, +/-sectors or +/-size{K,M,G,T,P} (5390336-123596799, default 123596799): 123596799

Created a new partition 3 of type 'Linux' and of size 56.4 GiB.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.
    Check the block device information again to make sure the new partition has been created. 
    thistle@rpi4-util:~ $ lsblk /dev/mmcblk0
NAME        MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
mmcblk0     179:0    0 58.9G  0 disk
├─mmcblk0p1 179:1    0  512M  0 part
├─mmcblk0p2 179:2    0  2.1G  0 part
└─mmcblk0p3 179:3    0 56.4G  0 part
  4. We also need to customize the file system table (the fstab file) of the microSD card’s default rootfs, and add the tuc binary to the rootfs.
    • Mount the default rootfs partition. 
      thistle@rpi4-util:~ $ mkdir mymount
thistle@rpi4-util:~ $ sudo mount /dev/mmcblk0p2 mymount/
    • Modify /etc/fstab 
      thistle@rpi4-util:~ $ sudo nano mymount/etc/fstab
      Find the line 
      PARTUUID=...  /boot/firmware  vfat    defaults          0       2
      Change the second column from /boot/firmware to /boot. Save and exit.
    • Add the latest TUC binary to /usr/bin/ 
      thistle@rpi4-util:~ $ wget https://downloads.thistle.tech/embedded-client/1.5.0/tuc-1.5.0-aarch64-unknown-linux-musl.gz
thistle@rpi4-util:~ $ gunzip tuc-1.5.0-aarch64-unknown-linux-musl.gz
thistle@rpi4-util:~ $ chmod +x tuc-1.5.0-aarch64-unknown-linux-musl
thistle@rpi4-util:~ $ sudo cp tuc-1.5.0-aarch64-unknown-linux-musl mymount/usr/bin/tuc
    • Sync the changes made to the microSD card, and umount. 
      thistle@rpi4-util:~ $ sync
thistle@rpi4-util:~ $ sudo umount mymount
    • For convenience of the demo, enable UART in the demo OS by modifying config.txt in the boot partition. 
      thistle@rpi4-util:~ $ sudo mount /dev/mmcblk0p1 mymount/
thistle@rpi4-util:~ $ sudo nano mymount/config.txt
      Add enable_uart=1 to the last line of config.txt. Save the change, and power off the Raspberry Pi. 
      thistle@rpi4-util:~ $ sync
thistle@rpi4-util:~ $ sudo umount mymount
thistle@rpi4-util:~ $ sudo poweroff
    Now the microSD card is ready for the A/B update demo.

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OTA A/B Update Demo

When the RPi-4 is powered off, unplug the USB drive, and leave the microSD card inserted. On PC, connect to the serial port so you can start watching boot logs. 
# This command runs on a Linux PC. Change /dev/ttyUSB0 to your serial port if
# needed.
$ minicom -b 115200 -D /dev/ttyUSB0
Now power on the RPi-4 board. This time it boots from the microSD card. You should see boot logs from the serial console, and eventually the login prompt. Verify that you can login with the username and password you set when programming the microSD card. 
thistle@ota-demo:~ $ uname -a
Linux ota-demo 6.12.25+rpt-rpi-v8 #1 SMP PREEMPT Debian 1:6.12.25-1+rpt1 (2025-04-30) aarch64 GNU/Linux

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Create an update bundle on PC

For the purpose of this example, we will release the alternative rootfs image as an update. This image will be deployed with the Release Helper, and will be installed on the target automatically by the Update Client. First download the rootfs update image to PC. 
$ curl -O https://downloads.thistle.tech/rpi/four-v1.5.0-updated.rootfs
Let’s start with packaging the update: 
# Set up your project's access token - Bash, on Linux/Unix or Windows Subsystem for Linux (WSL)
$ export THISTLE_TOKEN=$(cat)
(paste access token, press enter, then ctrl-d)

# Set up your project's access token - Windows PowerShell
$ $env:THISTLE_TOKEN = "[Access Token Obtained from Thistle App's Projects Section]"

$ ./trh --signing-method="remote" init -persist="/boot"

# prepare manifest and deploy partition image
$ ./trh --signing-method="remote" prepare --target=./four-v1.5.0-updated.rootfs

$ ./trh --signing-method="remote" release
The command line option --signing-method="remote" uses a Thistle-managed, Cloud KMS-backed ECDSA-P256 key to sign OTA update bundles. For other signing methods supported by TRH, please refer to our blog post OTA Bundle Signing in Production with Thistle Release Helper.

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Amend device configuration for bootloader

The Thistle Update Client natively supports the Raspberry Pi bootloader. The only step required is to specify the bootloader as RaspberryPi and the two partitions that will be used as root filesystem alternatively. Amend your configuration using the snippet below as the microSD card partitions were specified during the image generation. 
$ cat config.json
{
    ...
    "bootloader": "RaspberryPi",
    "part_a": "/dev/mmcblk0p2",
    "part_b": "/dev/mmcblk0p3"
}
It is now required to copy config.json onto the Raspberry Pi, this can be done using scp or using a USB key. This is necessary so that we can run the update client on the Raspberry Pi with the appropriate configuration. Under normal circumstances, this operation will be performed automatically during e.g. the image personalization step of a deploy pipeline. 
# on the PC (host machine) - you likely need to amend the IP address
scp config.json thistle@192.168.1.102:~/tuc-config.json
We are now ready to run the client on the Raspberry Pi, and observe our first update cycle.

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Test deployed release

We can now test our deployed release. Note that as the assets will be downloaded from the Thistle Backend Server, an internet connection is required. 
# on the Raspberry Pi - confirm tuc-config.json file is present
thistle@ota-demo:~$ cat ~/tuc-config.json

# `mmcblk0p2` is currently used as mountpoint for root
thistle@ota-demo:~$ mount | grep /dev/mmc
/dev/mmcblk0p2 on / type ext4 (rw,relatime)

# we copy `tuc-config.json` to the /boot directory so that it's persistent
# across updates
thistle@ota-demo:~$ sudo cp ~/tuc-config.json /boot
Let’s apply the update and see the result. 
thistle@ota-demo:~$ sudo tuc -c /boot/tuc-config.json
The Raspberry Pi will now reboot. Login again and confirm that the appropriate partition is now the root partition.
The default username and password for this alternative rootfs image are thistle and raspberry, respectively.
# `mmcblk0p3` is now used as mountpoint for root
raspberrypi4-64-thistle:~$ mount | grep /dev/mmc
/dev/mmcblk0p3 on / type ext4 (rw,relatime)

# this marker file should appear in /home/thistle with the expected content
raspberrypi4-64-thistle:~$ cat message_from_thistle.txt
If you can see this message, your root filesystem has been successfully OTA
updated.

# latch in update by re-running tuc
raspberrypi4-64-thistle:~$ sudo tuc -c /boot/tuc-config.json
You just performed your first automated A/B update cycle 🎉 Have a look at the Thistle App, your device can now be monitored directly there.

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More options

This tutorial shows how to manually perform an update on the Raspberry Pi. This process can be automated to update a fleet of devices. In this regards, we support different enrollment setups, either based on Trust On First Use (TOFU), or only accepting pre-enrolled devices.