Writable storage overlay steps
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extroot.txt
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182
extroot.txt
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OpenWrt 24.x Extroot on XE300
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Goal: Use a 512 GB microSD card as the router’s writable storage (overlay),
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single ext4 partition, no data copied from internal flash.
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Assumptions (based on your box):
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Device: GL‑iNet XE300
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OpenWrt 24.x, fresh flash
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SD card in the built‑in slot
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The card appears as /dev/sda with one partition /dev/sda1
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You don’t care about any existing data on the card or in the old overlay
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1. Insert card & install required packages
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Insert the microSD card.
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SSH into the router (root@192.168.x.x etc).
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Install the packages that make extroot possible:
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opkg update
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opkg install block-mount kmod-fs-ext4 kmod-usb-storage e2fsprogs
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What they do:
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block-mount – block info, fstab handling, extroot logic
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kmod-fs-ext4 – kernel driver for ext4 filesystem
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kmod-usb-storage – makes the SD slot show up as /dev/sda
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e2fsprogs – gives you mkfs.ext4
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2. Confirm the SD card is detected as /dev/sda//dev/sda1
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Check dmesg:
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dmesg | grep -i sd
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On your XE300 you should see something like:
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sd 0:0:0:0: [sda] 1000005632 512-byte logical blocks: (512 GB/477 GiB)
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sda: sda1
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sd 0:0:0:0: [sda] Attached SCSI removable disk
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That tells you:
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Disk: /dev/sda
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Partition already present: /dev/sda1
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If you don’t see sda1 for some reason in the future, you’d need to create a partition—but on your current setup, it’s there already, so we can jump straight to formatting.
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3. Format /dev/sda1 as ext4 for extroot
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We’re going to erase whatever is on that partition and build a fresh ext4 FS.
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mkfs.ext4 -L extroot /dev/sda1
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You may get a warning like “contains an exfat file system – Proceed anyway? (y,N)”
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Answer y
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You should see output about creating the filesystem, ending with success and a UUID.
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At this point /dev/sda1 is a clean ext4 filesystem labeled extroot.
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4. Grab the UUID and create the extroot fstab entry
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Ask OpenWrt’s block layer for info and capture the UUID:
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UUID=$(block info | awk -F\" '/sda1/ {print $2}')
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echo $UUID
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You should see a UUID like:
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4851f00e-696b-47d3-a529-82c2aca292db
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Now write the minimal extroot config via UCI:
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uci set fstab.extroot="mount"
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uci set fstab.extroot.uuid="$UUID"
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uci set fstab.extroot.target="/overlay"
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uci set fstab.extroot.fstype="ext4"
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uci set fstab.extroot.enabled="1"
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uci commit fstab
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Sanity‑check the file:
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cat /etc/config/fstab
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Expected content (plus the global section):
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config global
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option anon_swap '0'
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option anon_mount '0'
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option auto_swap '1'
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option auto_mount '1'
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option delay_root '5'
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option check_fs '0'
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config mount 'extroot'
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option uuid '4851f00e-696b-47d3-a529-82c2aca292db'
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option target '/overlay'
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option fstype 'ext4'
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option enabled '1'
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Note what’s not there:
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No /rwm mount
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No copy of internal overlay
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This is intentionally a clean, single‑overlay setup that lives entirely on the SD card.
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5. Reboot into extroot
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reboot
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Let the router come back up and SSH in again.
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6. Verify that the SD card is now your root overlay
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Run:
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df -h
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On your working system, you got:
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Filesystem Size Used Available Use% Mounted on
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/dev/root 4.0M 4.0M 0 100% /rom
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tmpfs 58.7M 200.0K 58.5M 0% /tmp
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/dev/sda1 468.3G 2.1M 444.4G 0% /overlay
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overlayfs:/overlay 468.3G 2.1M 444.4G 0% /
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Interpreting that:
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/dev/root at /rom → the read‑only squashfs from flash (normal)
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/dev/sda1 mounted at /overlay → SD card is the writable layer
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overlayfs:/overlay mounted on / → your whole root filesystem is now the ROM + SD overlay
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That’s exactly what we want: the router is effectively a 468 GB OpenWrt box.
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Extra checks if you ever want them:
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mount | grep sda1
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mount | grep overlay
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Should show:
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/dev/sda1 on /overlay type ext4
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overlayfs:/overlay on / type overlay
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7. Quick functional test
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Just to flex:
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opkg install tcpdump
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df -h /overlay
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You’ll see a tiny bump in “Used” on /dev/sda1, confirming packages are being installed to SD‑backed overlay, not the tiny internal flash.
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