How to use overlayfs on Linux

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1 What Is OverlayFS

OverlayFS is a union mount filesystem implementation for Linux. It allows a virtual merge of two partitions, while keeping their actual contents separate. One partition is the rootfs partition and the other is the data partition. It has the following advantages:
1) you can easily restore a system's factory settings by formatting the data partition;
2) you can still boot your system since the rootfs is read-only even when the data partition cannot be correctly mounted due to unexpected shutdown.

2 FriendlyELEC's Systems That Support OverlayFS

2.1 Hardware Systems

H3, H5, S5P4418, S5P6818, RK3399, RK3328, RK3568, RK3588 based boards

2.2 OS Systems

All Linux-based systems

2.3 Scope of Application

This document is only applicable to products using Rockchip platform. If you are using other platforms, please click on this link: How to use overlayfs on S5Pxxxx,H3,H5 platform

3 How to Check Whether OverlayFS Is Working

Run the df command. If the "/" partition is mounted as "overlay" it means OverlayFS is working;

pi@NanoPi-R6C:/etc$ df -h
Filesystem      Size  Used Avail Use% Mounted on
tmpfs           792M  2.2M  790M   1% /run
overlay          25G   13G   11G  53% /
tmpfs           3.9G     0  3.9G   0% /dev/shm
tmpfs           5.0M  4.0K  5.0M   1% /run/lock
tmpfs           793M  116K  793M   1% /run/user/1000

4 Partition Settings With OverlayFS

If OverlayFS is enabled in your system there will be three partitions:rootfs, data and boot, and these partitions correspond to a rootfs.img, a userdata.img and a boot.img separately:

pi@NanoPi-R6C:/etc$ sudo apt install parted
pi@NanoPi-R6C:/etc$ sudo parted /dev/mmcblk0 print
Model: SD SR32G (sd/mmc)
Disk /dev/mmcblk0: 31.9GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Disk Flags:
 
Number  Start   End     Size    File system  Name      Flags
 1      8389kB  12.6MB  4194kB               uboot
 2      12.6MB  16.8MB  4194kB               misc
 3      16.8MB  21.0MB  4194kB               dtbo
 4      21.0MB  37.7MB  16.8MB               resource
 5      37.7MB  79.7MB  41.9MB               kernel
 6      79.7MB  113MB   33.6MB               boot
 7      113MB   147MB   33.6MB               recovery
 8      147MB   4173MB  4027MB  ext4         rootfs
 9      4173MB  31.9GB  27.7GB  ext4         userdata

The system data will be in the rootfs partition and the data written to the root directory will be in the data partition.

5 Restore Factory Settings With OverlayFS

5.1 Method 1: Operate in Userdata Partition

factory reset:

mount /dev/mmcblk0p3 /media/ && cd /media/
mv root/ deleteme.root
mv work/ deleteme.work
reboot

cleanup after reboot:

mount /dev/mmcblk0p3 /media/ && rm -rf /media/deleteme.*

5.3 S5P4418 & S5P6818 platform

Add "wipedata=yes" to the kernel's commandline parameters. For S5P4418 and S5P6818 based boards you can do it after system boots;

5.3.1 恢复出厂设置

sudo fw_setenv bootargs "`sed 's/ wipedata=yes//g;s/ wipedata=no//g' /proc/cmdline` wipedata=yes"
sudo reboot

5.3.2 Keep Data in Data Partition on System Reboot

sudo fw_setenv bootargs "`sed 's/ wipedata=yes//g;s/ wipedata=no//g' /proc/cmdline` wipedata=no"
sudo reboot

For other SoC based boards you need to make changes in the kernel commandline by following the methods supported by their BSPs or by entering uboot to proceed.

5.3.3 RK3399 platform

echo wipedata=yes > /var/.init_wipedata; sync; reboot

The next time you boot to the ramdisk stage, the data partition will be formatted, and the /var/.init_wipedata file will also be deleted.

5.3.4 H3/H5 platform

5.3.4.1 change boot.cmd

Enter the boot directory and modify file boot.cmd, change the following:

setenv bootargs "console=${debug_port} earlyprintk
root=/dev/mmcblk0p2 rootfstype=ext4 rw rootwait fsck.repair=${fsck.repair}
panic=10 fbcon=${fbcon} ${hdmi_res} ${overlayfs} ${pmdown}"

to:

setenv bootargs "console=${debug_port} earlyprintk
root=/dev/mmcblk0p2 rootfstype=ext4 rw rootwait fsck.repair=${fsck.repair}
panic=10 fbcon=${fbcon} ${hdmi_res} ${overlayfs} ${pmdown} wipedata=yes"

Added wipedata=yes.

5.3.4.2 regenerate boot.scr

Enter the boot directory，execute the following command：

$ mkimage -C none -A arm -T script -d boot.cmd boot.scr

6 Disable OverlayFS

If you have an installation TF card, use the fdisk command to delete the data partition.