The wiki has recently changed, but it looks like here are some stuff : https://openwrt.org/docs/guide-user/additional-software/extroot_configuration
For me, I have a 4Gb eMMC, and I used firmware from OpenWrt which do not fill the eMMC...
So I use an extend (third) partition, to clone my rootfs on it...
here are from my notes some tweaks :
prepare the disk partitions :
opkg install fdisk
prepare the root partion :
prepare the root file system :
clone the rootfs content to the new rootfs :
mkdir -p /tmp/cproot
mount --bind / /tmp/cproot
mount /dev/mmcblk0p3 /tmp/mmcblk0p3
tar -C /tmp/cproot -cvf - . | tar -C /tmp/mmcblk0p3 -xf -
install the block-mount (luci and uci)
opkg install block-mount
add you're new partition for the rootfs
option enabled '1'
option enabled_fsck '1'
option target '/'
option device '/dev/mmcblk0p3'
then reboot and you'll get a ReadOnly rootfs in /rom and a Read Write rootfs on /
The both will be auto repaired in cased of crash, hard reset or switchoff...
verify your filesystems status with
At Upgrade, you'll need to reinstall fdisk, kmod-fs-ext4, block-mount, if needed.
Then you'll have to clone again the new upgraded filesystems (/rom) to the third partition rootfs !
You may need to do this from recovery mode, or to disable /rootfs and clone it before re-enable it !
keep of configs while upgrading may confuse the system, and need some manual tweaks...
Hope this will help...