OpenWrt A/B Upgrades

Overview

A/B upgrades allow for safer OpenWrt upgrades. If the upgrade does not go as planned, you can easily boot into your previous image.

The A/B upgrade approach functions by having two root parititons. When executing an upgrade the new image will be written to the non-active root partition. The grub config is updated to allow booting into either image.

At boot, you'll be able to select between either image. The most recently installed/upgraded image will be the default.

• OpenWrt SNAPSHOT r15199-5d2b577a53
• OpenWrt SNAPSHOT r15129-d346beb08c

Partition Layout

• /dev/sda1 = /boot
• /dev/sda2 = rootfs_a
• /dev/sda3 = rootfs_b

High Level Process

1. Detect non-active partition
2. Create sysupgrade backup
3. Format partition
4. Mount partition to /mnt/sysupgrade
5. Extract rootfs to /mnt/sysupgrade
6. Copy kernel to /mnt/sysupgrade/vmlinuz
7. Extract openwrt-backup to /mnt/sysupgrade
8. Backup grub.cfg
9. Generate new grub.cfg
10. Reboot

Setup

1. Install Packages: opkg install fdisk coreutils-stat blkid e2fsprogs
2. Create partiton for second rootfs fdisk /dev/sda

   root@OpenWrt:~# fdisk /dev/sda
   
   Command (m for help): p
   Disk /dev/sda: 8 GiB, 8589934592 bytes, 16777216 sectors
   Device     Boot Start    End Sectors  Size Id Type
   /dev/sda1  *      512  33279   32768   16M 83 Linux
   /dev/sda2       33792 558079  524288  256M 83 Linux
   
   Command (m for help): n
   Select (default p): p
   Partition number (3,4, default 3): 3
   First sector (33280-16777215, default 559104):
   Last sector, +/-sectors or +/-size{K,M,G,T,P} (559104-16777215, default 16777215): +256M
   
   Created a new partition 3 of type 'Linux' and of size 256 MiB.
   
   Command (m for help): w
   The partition table has been altered.
   Syncing disks.
   

3. Copy current kernel to rootfs: cp /boot/vmlinuz /vmlinuz

Upgrading

1. Modify upgrade.sh
   • Update BOOT_DEV, ROOTA_DEV, ROOTB_DEV
   • Update default grub options
2. Install Packages: opkg install coreutils-stat blkid
3. Download rootfs and kernel to /root

   wget https://downloads.openwrt.org/snapshots/targets/x86/64/openwrt-x86-64-generic-kernel.bin
   wget https://downloads.openwrt.org/snapshots/targets/x86/64/openwrt-x86-64-rootfs.tar.gz
   

4. Upgrade: ./upgrade.sh openwrt-x86-64-generic-kernel.bin openwrt-x86-64-rootfs.tar.gz
5. Reboot