Ubuntu - Boot to RAM - Couple of notes

This post contains couple of notes on this topic in form of very short guide.

Requirements

First step - create virtual machine

Create new virtual machine using VirtualBox and install minimal Ubuntu OS.

Second step - install required packages

Install squashfs-tools package so you can create squashfs image later:

$ sudo apt-get install squashfs-tools

Install live-boot package with dependencies (live-boot-initramfs-tools) so you can use boot to ram:

$ sudo apt-get install live-boot

Third step - prepare image contents

Create new directory and copy root file system contents:

$ sudo mkdir /squashfs

You need to exclude contents of directory /live and one created just moment ago.

You can also exclude contents of directories like /boot/*, /tmp/*, ...

$ sudo rsync -a --delete --one-file-system / /squashfs \
    --exclude=/live --exclude=/squashfs

It's good idea to remove root file system from /squashfs/etc/fstab file now.

Fourth step - create squashfs image

Create /live directory (squashfs image will be stored here):

$ sudo mkdir /live

Create squashfs image:

$ sudo mksquashfs /squashfs /live/livefs.squashfs -noappend -always-use-fragments

Fifth step - configure grub2

Change GRUB_TIMEOUT to -1 in /etc/defaults/grub so it will be waiting forever in boot menu.

Update grub configuration:

$ sudo update-grub

Check your kernel release:

$ uname -r
3.0.0-17-generic

Edit /etc/grub.d/40_custom file to add new entry in grub2 menu and take into account your kernel release:

menuentry "Live minimal OS" {
 set root='(hd0,1)'
 linux /boot/vmlinuz-3.2.0-24-generic boot=live toram=livefs.squashfs
 initrd /boot/initrd.img-3.2.0-24-generic
}

Update grub configuration again:

$ sudo update-grub

Sixth step - check it out!

Reboot system and check it out:

$ sudo reboot

Couple of notes

Compression is very effective as it can compress 2 GB file system (KDE + Libre Office + couple of smaller applications) to around 700 MB.

System becomes blazingly fast and you can easily create live usb this way.

To make your changes persistent just create partitions labelled accordingly live-rw for root, home-rw for home file system and add persistent parameter to kernel boot parameters. If you can't create new partitions then create files in root directory (not in live fs). For example to create persistent home (200MB, without reserved blocks) use commands:

$ sudo dd if=/dev/zero of=/home-rw bs=1M count=200
$ sudo mkfs.ext4 /home-rw
$ sudo tune2fs -m 0 /home-rw
$ sudo tune2fs -L home-rw /home-rw

Apparmor doesn't work very well so you need to remove it.

To see how it works go to directory /usr/share/initramfs-tools/scripts and start with reading live file. After any modifications don't forget to update initramfs:

$ sudo update-initramfs -u

Don't forget to read live-boot manual page.

Couple of common errors

only one RO file system supported with exposedroot

Just remove livefs.squashfs file from /squashfs/live/ directory and create squashfs image again.

a wrong rootfs was mounted
This error means that live-boot package was not installed on system that was used to create squashfs image. Just install it and create image again.
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About Milosz Galazka

Milosz is a system administrator working for a successful Polish company and a long time supporter of Free Software Foundation and Debian operating system.

Gdansk, Poland https://sleeplessbeastie.eu