OpenWRT Pivot overlay to external USB drive

I have tested this pivot overlay on TP-LINK WR1043ND v2.1 router

1. Compile custom firmware image as described in my previous post. For enabling usb select following packages in menuconfig.

kernel modules -> usb support -> kmod-usb-storage, kmod-usb-storage-extras, kmod-usb-uhci (If needed, ohci is selected by default)

kernel modules -> filesystems -> kmod-fs-ext4

Utilities -> Filesystem -> e2fsprogs

Utilities -> disc -> fdisk

Utilities -> usbutils

Base system -> block-mount

Alternately these packages can be installed using 'opkg install'

2. Create the linux partitions using fdisk as per your need

fdisk /dev/sda

m - Help
p - to print partition
n - new partition
d - delete partition

My partitions for example -

fdisk -l /dev/sda

Disk /dev/sda: 3.8 GiB, 4009754624 bytes, 7831552 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: 0x00000000

Device    Boot     Start       End  Blocks  Id System
/dev/sda1           2048   7362559 3680256  83 Linux
/dev/sda2        7362560   7831551  234496  82 Linux swap / Solaris

3. Format the partition with ext4

mkfs.ext4 /dev/sda1

Now mount the filesystem

mkdir /mnt/sda1
mount /dev/sda1 /mnt/sda1

4. Copy contents from /overlay to usb device
tar -C /overlay -cvf - . | tar -C /mnt/sda1 -xf -

5. Generate and edit fstab file
block detect > /etc/config/fstab
vi /etc/config/fstab

change the target to '/overlay' & enabled option to '1'. Do not change UUID or other settings.

6. Restart the router.

7. Verify the mount after reboot (My output shown below).

Filesystem                Size      Used Available Use% Mounted on
rootfs                    3.4G      7.2M      3.2G   0% /
/dev/root                 3.0M      3.0M         0 100% /rom
tmpfs                    30.0M     48.0K     30.0M   0% /tmp
/dev/sda1                 3.4G      7.2M      3.2G   0% /overlay
overlayfs:/overlay        3.4G      7.2M      3.2G   0% /
tmpfs                   512.0K         0    512.0K   0% /dev

8. Block mount also detects other settings, fstab file after reboot with detected settings is shown below:

config 'global'
option anon_swap '0'
option anon_mount '0'
option auto_swap '1'
option auto_mount '1'
option delay_root '35'
option check_fs '0'

config 'mount'
option target '/overlay'
option uuid '4367f74a-86e8-4340-b8d2-fd458433f9d0'
option fstype 'ext4'
option options 'rw,sync'
option enabled_fsck '0'
option enabled '1'

References:
http://wiki.openwrt.org/doc/howto/extroot
http://wiki.openwrt.org/doc/uci/fstab