Convert Raspberry Pi Ubuntu to iSCSI btrfs root

This is to move root file system of Raspberry Pi Ubuntu OS to iSCSI LUN and convert it to btrfs file system

Pros

• Cheaper than using small SD card
• Backup and restore easy, only need to backup 150MB boot partition on SD card
• Able to perform snapshot at LUN level or OS level (btrfs)
• Should be Faster

Steps

Install iscsi packages

apt install open-iscsi
systemctl enable open-iscsi
systemctl start open-iscsi
systemctl enable iscsid
systemctl start iscsid

Configure iscsi

Edit /etc/iscsi/initiatorname.iscsi, update following line

InitiatorName=<YOUR_INITIATOR_NAME>

Note: The YOUR_INITIATOR_NAME is the iSCSI client name

Edit /etc/iscsi/iscsid.conf, update following lines

node.session.auth.authmethod = CHAP
node.session.auth.username = <YOUR_USERNAME>
node.session.auth.password = <YOUR_PASSWORD>

# iscsiadm --mode discovery --type sendtargets --portal <YOUR_TARGET_IP>
# iscsiadm --mode node --targetname <YOUR_TARGET_NAME> --portal <YOUR_TARGET_IP> --login

Note: If can not login, restart iscsid and try again.

systemctl restart iscsid

Identify block device

Use lsblk command to identify device file, normally should be /dev/sda.

Partitioning

Creating two partitions using fdisk, the first partition is to prepare following for future used, such as

• Network boot
• UEFI iSCSI boot
• SD card backup

Partition /dev/sda1: vfat, 2GB
Partition /dev/sda2: for root filesystem

Create filesystems

mkfs.vfat /dev/sda1
mkfs.btrfs /dev/sda2

Identify UUID for root filesystem

blkid /dev/sda2

Update initramfs

This is to enable ubuntu load iscsi driver during boot

touch /etc/iscsi/iscsi.initramfs
update-initramfs -v -k $(uname -r) -c

Duplicate files

mount /dev/sda2 /mnt
rsync -avhP --exclude /boot/firmware --exclude /proc --exclude /sys --exclude /dev --exclude /mnt / /mnt/
mkdir /mnt/{dev,proc,sys,boot/firmware,mnt}

Modify /etc/fstab in LUN

Note: Don't modify the file in /etc, it will not be used during iSCSI boot.

vi /mnt/etc/fstab

Change root mounting to

UUID=<YOUR_DEV_UUID> / btrfs defaults 1 1

Modify /boot/firmware/cmdline.txt

First create a backup of this file

cp /boot/firmware/cmdline.txt /boot/firmware/cmdline.txt.sav

Change the content of /boot/firmware/cmdline.txt

Note: Beware of rootfstype=btrfs

net.ifnames=0 dwc_otg.lpm_enable=0 console=serial0,115200 console=tty1 rootfstype=btrfs elevator=deadline rootwait fixrtc ip=dhcp root=UUID=<YOUR_DEV_UUID> ISCSI_INITIATOR=<YOUR_INITIATOR_NAME> ISCSI_TARGET_NAME=<YOUR_TARGET_NAME> ISCSI_TARGET_IP=<YOUR_TARGET_IP> ISCSI_TARGET_PORT=3260 ISCSI_USERNAME=<YOUR_USERNAME> ISCSI_PASSWORD=<YOUR_PASSWORD> rw

Note: Root partition can also be identified by label if assigned one, see next topic

Reboot

umount /mnt
reboot

Use LABEL for root

Using label instead of UUID for root filesystem.

Assign a label

For mounted filesystem

sudo btrfs filesystem label <mountpoint> <newlabel>

For not mounted filesystem

sudo btrfs filesystem label <device> <newlabel>

Change /etc/fstab

LABEL=<newlabel> / btrfs defaults 1 1

Change /boot/fireware/cmdline.txt

... root=LABEL=<newlabel> ...

Use Static IP

To use static IP for iSCSI connection, the ip definition in cmdline.txt needs to be changed to

ip=192.168.1.200::192.168.1.1:255.255.255.0:rpi:eth0:off

This will create an IP 192.168.1.200 on interface eth0 as below.

2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000
    link/ether dc:a6:32:ef:07:0f brd ff:ff:ff:ff:ff:ff
    inet 192.168.1.200/24 brd 192.168.1.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet 192.168.1.23/24 brd 192.168.1.255 scope global secondary dynamic eth0
       valid_lft 43036sec preferred_lft 43036sec
    inet6 fe80::dea6:32ff:feef:70f/64 scope link
       valid_lft forever preferred_lft forever

One way to just maintain one IP is, disabling OS IP address.

Reboot

Other cmdline.txt format

I tried following format, but failed. I think it maybe work if no partition in iSCSI LUN

root=iscsi:[<username>:<password>[:<reverse>:<password>]@][<servername>]:[<protocol>]:[<port>][:[<iscsi_iface_name>]:[<netdev_name>]]:[<LUN>]:<targetname>

Use UUID is better, because there is no need to worry about LUN id and partition as UUID is unique ideitifier.

To specfic UUID, following format can be used according to kernel parameters, but it doesn't work too.

root=UUID=<UUID>
netroot=iscsi:[<username>:<password>[:<reverse>:<password>]@][<servername>]:[<protocol>]:[<port>][:[<iscsi_iface_name>]:[<netdev_name>]]:[<LUN>]:<targetname>

I also tried rd.* format, such as rd.iscsi.initiator, etc., they are new format to replace old format ISCSI_INITIATOR, etc., but also not failed. I think the dracut.cmdline version used by ubuntu in raspberry pi is old. Maybe should try grub2 used in Fedora OS for raspberry pi, or uefi used in Windows 10.

Backup and restore using pre-backup data

Backup data into iSCSI LUN partition 1

First create vfat in iSCSI LUN as partition 1, then backup /boot/fireware data into that partition

mkfs.vfat /dev/sda1
mount /dev/sda1 /mnt
cp /boot/firmware/. /mnt
umount /mnt

Restore to an empty SD card

Create partition in new SSD as type c, which W95 FAT32 (LBA), with Boot flag.

Device         Boot Start     End Sectors  Size Id Type
/dev/mmcblk0p1       2048 1050623 1048576  512M  c W95 FAT32 (LBA)

Format SD card and give label as system-boot, the LABEL is defined in /etc/fstab, it can be changed to UUID if needed.

mkfs.vfat -n system-boot /dev/sdb1
mount /dev/sdb1 /mnt
cp -a <backup_filesystem> /mnt
umount /mnt

Make sure system reported correct LABEL on the newly created vfat filesystem, using blkid command to verify.

Troubleshooting

iscsi_tcp missing

If following error occurred, install package linux-modules-extra.

libkmod: ERROR ../libkmod/libkmod-module.c:838 kmod_module_insert_module: could not find module by name='iscsi_tcp'

Please read post Missing iSCSI module in Ubuntu 20.10

Reboot error

If failed to boot, initram command prompt will appear. In this case, following commands can be used to recover back the booting.

mkdir /mnt
mount /dev/mmcblk0p1 /mnt
cd /mnt
cp cmdline.txt.sav cmdline.txt
cd /
umount /mnt
reboot

Note: the umount is very important, otherwise, the changes wouldn't be saved.

There are very minimum commands can be used, such as no vi. So let it boots into previous status, then troubleshooting from there.

References

dracut kernel command line options
Kernel command line parameters
introduction to boot time parameters of the Linux kernel
Raspberry Pi 4 UEFI Boot
RPi cmdline.txt
RPi config.txt
kernel-parameters.txt
The config.txt file
Raspberry Pi iSCSI Root on Ubuntu 20.04
btrfs root filesystem on raspbian
[Howto] booting from iSCSI
Ubuntu Server 20.10 on Raspberry Pi 4: installation guide with USB Boot (no SD card) and full disk encryption (excluding /boot) using btrfs-inside-luks and auto-apt snapshots with Timeshift
Raspberry Pi 4 - Ubuntu 20.04 w/Btrfs root
dracut.cmdline(7) — Linux manual page