Posted on Feb 16, 2019 By copyninja under devops

These 2 are the use cases I came across for expanding root file system.

  1. RaspberryPi images which comes in smaller image size like 1GB which you write bigger size SD cards like 32GB but you want to use full 32GB of space when system comes up.
  2. You have a VM image which is contains basic server operating system and you want to provision the same as a VM with much larger size root file system.

My current use case was second but I learnt the trick from 1, that is the RaspberryPi3 image spec by Debian project.

Idea behind the expanding root file system is first expanding the root file system to full available size and then run resize2fs on the expanded partition to grow file system. resize2fs is a tool specific for ext2/3/4 file system. But this needs to be done before the file system is mounted.

Here is my modified script from raspi3-image-spec repo. Only difference is I've changed the logic of extracting root partition device to my need, and of course added comments based on my understanding.


# Just extracts root partition and removes partition number to get the device
# name eg. /dev/sda1 becomes /dev/sda
roottmp=$(lsblk -l -o NAME,MOUNTPOINT | grep '/$')
rootpart=/dev/${roottmp%% */}

# Use sfdisk to extend partition to all available free space on device.
flock $rootdev sfdisk -f $rootdev -N 2 <<EOF

sleep 5

# Wait for all pending udev events to be handled
udevadm settle

sleep 5

# detect the changes to partition (we extended it).
flock $rootdev partprobe $rootdev

# remount the root partition in read write mode
mount -o remount,rw $rootpart

# Finally grow the file system on root partition
resize2fs $rootpart

exit 0fs

raspi3-image-spec uses sytemd service file to execute this script just before any file system is mounted. This is done by a making service execute before local-fs.pre target. From the man page for systemd.special
systemd-fstab-generator(3) automatically adds dependencies of type Before= to all mount units that refer to local mount points for this target unit. In addition, it adds dependencies of type Wants= to this target unit for those mounts listed in /etc/fstab that have the auto mount option set.

Service also disables itself on executing to avoid re-runs on every boot. I've used the service file from raspi3-image-spec as is.

Testing with VM

raspi3-image-spec is well tested, but I wanted to make sure this works with my use case for VM. Since I didn't have any spare physical disks to experiment with I used kpartx with raw file images. Here is what I did

  1. Created a stretch image using vmdb2 with grub installed. Image size is 1.5G
  2. I created another raw disk using fallocate of 4G size.
  3. I created a partition on 4G disk.
  4. Loop mounted the disk and wrote 1G image on it using dd
  5. Finally created a VM using virt-install with this loop mounted device as root disk.

Below is my vmdb configuration yml again derived from raspi3-image-spec one with some modifications to suit my needs.

# See for known issues and more details.
- mkimg: "{{ output }}"
  size: 1.5G

- mklabel: msdos
  device: "{{ output }}"

- mkpart: primary
  device: "{{ output }}"
  start: 0%
  end: 100%
  tag: /

- kpartx: "{{ output }}"

- mkfs: ext4
  partition: /
  label: RASPIROOT

- mount: /

- unpack-rootfs: /

- debootstrap: stretch
  mirror: http://localhost:3142/
  target: /
  variant: minbase
  - main
  - contrib
  - non-free
  unless: rootfs_unpacked

# TODO( remove this workaround once
# debootstrap is fixed
- chroot: /
  shell: |
    echo 'deb buster main contrib non-free' > /etc/apt/sources.list
    apt-get update
  unless: rootfs_unpacked

- apt: install
  - ssh
  - parted
  - dosfstools
  - linux-image-amd64
  tag: /
  unless: rootfs_unpacked

- grub: bios
  tag: /

- cache-rootfs: /
  unless: rootfs_unpacked

- shell: |
     echo "experimental" > "${ROOT?}/etc/hostname"

     # '..VyaTFxP8kT6' is crypt.crypt('raspberry', '..')
     sed -i 's,root:[^:]*,root:..VyaTFxP8kT6,' "${ROOT?}/etc/shadow"

     sed -i 's,#PermitRootLogin prohibit-password,PermitRootLogin yes,g' "${ROOT?}/etc/ssh/sshd_config"

     install -m 644 -o root -g root fstab "${ROOT?}/etc/fstab"

     install -m 644 -o root -g root eth0 "${ROOT?}/etc/network/interfaces.d/eth0"

     install -m 755 -o root -g root rpi3-resizerootfs "${ROOT?}/usr/sbin/rpi3-resizerootfs"
     install -m 644 -o root -g root rpi3-resizerootfs.service "${ROOT?}/etc/systemd/system"
     mkdir -p "${ROOT?}/etc/systemd/system/systemd-remount-fs.service.requires/"
     ln -s /etc/systemd/system/rpi3-resizerootfs.service "${ROOT?}/etc/systemd/system/systemd-remount-fs.service.requires/rpi3-resizerootfs.service"

     install -m 644 -o root -g root rpi3-generate-ssh-host-keys.service "${ROOT?}/etc/systemd/system"
     mkdir -p "${ROOT?}/etc/systemd/system/"
     ln -s /etc/systemd/system/rpi3-generate-ssh-host-keys.service "${ROOT?}/etc/systemd/system/"
     rm -f ${ROOT?}/etc/ssh/ssh_host_*_key*

  root-fs: /

# Clean up archive cache (likely not useful) and lists (likely outdated) to
# reduce image size by several hundred megabytes.
- chroot: /
  shell: |
     apt-get clean
     rm -rf /var/lib/apt/lists

# TODO( remove once vmdb
# clears /etc/resolv.conf on its own.
- shell: |
     rm "${ROOT?}/etc/resolv.conf"
  root-fs: /

I could not run with vmdb2 installed from Debian archive, so I cloned raspi3-image-spec and used vmdb2 submodule from it. And here are rest of commands used for testing the script.

fallocate -l 4G rootdisk.img
# Create one partition with full disk
sfdisk -f rootdisk.img <<EOF
kpartx -av rootdisk.img # mounts on /dev/loop0 for me
dd if=vmdb.img of=/dev/loop0
sudo virt-install --name experimental --memory 1024 --disk path=/dev/loop0 --controller type=scsi,model=virtio-scsi --boot hd --network bridge=lxcbr0

Once VM booted I could see the root file system is 4G of size instead of 1.5G it was after using dd to write image on to it. So success!.