Btrfs: what I miss?

ginerv33 · 15 November 2025 00:37

Hi, I want to use btrfs instead of Ext4 on latest OSMC for Raspberry pi4.

I have made those steps

a) made a backup with rsync

b)format my usb disk with btrfs

mkfs.btrfs /dev/sdb2

c)create subvol @

mount/dev/sdb2 /mnt

cd /mnt

btrfs subvol create @

cd

umount /mnt

mount -o subvol=@ /dev/sdb2 /mnt

d)restore backup on /mnt with rsync

e)edit config.txt and cmdline.txt

config.txt

arm_64bit=1
disable_fw_kms_setup=1
disable_overscan=1
disable_splash=1
dtoverlay=rpivid-v4l2
dtoverlay=vc4-kms-v3d,cma-256
dtoverlay=vc4-kms-v3d,cma-512
hdmi_ignore_cec_init=1
include config-user.txt
initramfs initrd.img-5.15.92-1-osmc followkernel
kernel=kernel.img
root=/dev/sda2 rootfstype=btrfs initrd=/boot/initrd.img-5.15.92-1-osmc rootwait quiet osmcdev=rbp4 rootflags=subvol=@
start_x=1

cmdline.txt

root=/dev/sda2 rootflags=subvol=/@ rootfstype=btrfs initrd=/boot/initrd.img-5.15.92-1-osmc rootwait quiet osmcdev=rbp4

What I miss?

When I start the device appear the “initramfs console” and said it cannot mount (no such device)

the device, but If I try to mount it manually works. It also give error about bad superblock on ext4 (but is not ext4).

What I miss?

sam_nazarko · 15 November 2025 01:10

Can you take a photo of the console?

I thought from previous posts you had it working.

You may need to adjust init= because our init will still try and mount ext4.

Out of curiosity, how big is your initramfs (du -h output please). Maybe we should look at using the Debian one for Raspberry Pi and adjusting our initramfs to include our helper scripts instead.

ginerv33 · 15 November 2025 01:43

du -h initrd.img-5.15.92-1-osmc
9.6M initrd.img-5.15.92-1-osmc

ginerv33 · 15 November 2025 01:44

No, is not working. It arrive on console and said the error message, manual mount works, but on reboot same error (try to check ext4, exit with errors, etc)

sam_nazarko · 15 November 2025 02:04

Thanks, that’s quite large. Possibly best avoided. Not sure why it needs to be that large and I’m sure it could be reduced.

See:

github.com/osmc/osmc

package/kernel-osmc/initramfs-src/init

master

#!/bin/busybox sh

# (c) 2014-2015 Sam Nazarko
# email@samnazarko.co.uk

# Abstract settings and defaults

install_symlinks()
{
	# Installs BusyBox symlinks to make the shell actually usable
	/bin/busybox --install -s
}

busybox_shell()
{
	install_symlinks
        if [ ! -z $OPTION_SERIAL_PORT ]
        then
                /bin/busybox getty -n -L $OPTION_SERIAL_PORT 115200 vt100 &
        fi

This file has been truncated. show original

and

github.com/osmc/osmc

package/kernel-osmc/initramfs-src/init.d/rbp464

master

#!/bin/busybox sh

export OPTION_DO_FSCK="1"
export OPTION_FORCE_FSCK="0"
export OPTION_ROOT="/dev/mmcblk0p2"
export OPTION_ROOTDELAY="0"
export OPTION_FILESYSTEM="ext4"
export OPTION_INIT="/sbin/splash_early"
export OPTION_MOUNT_OPTIONS=""
export OPTION_MOUNT_PATH="/real_root"
export OPTION_SERIAL_PORT="/dev/ttyAMA0"

I am happy to add btrfs support to our initramfs and think that is the best approach.

But where is your boot failing, can you please show me a photo of where it fails as requested previously?

If you are using your own initramfs, this should not be firing unless it hands over to our initramfs. But I would not expect this behaviour.

If this is the case, I’d suggest the following:

Introduce some flag so we don’t try and remount the rootfs, can just pass something in your initramfs like ‘alreadymounted’ to cmdline so we just switch_root directly.
Support btrfs directly (would do this first).

Because you’re passing parameters like ``rootflags=subvol=@``, I’d like to support btrfs directly instead. I know btrfs has excellent snapshot capabilities but wasn’t aware that it had significant advantages on devices like RBP with SD backed storage.

Sam

ginerv33 · 15 November 2025 03:32

Give me some minutes/hours, I will show a pic or better a movie showing fail

ginerv33 · 15 November 2025 03:43

Sorry for bad quality I hope is readable

As you can see it fail, but I can mount manually fs!

After do “exit” give me black screen