Difference between revisions of "Creating Arch Linux disk image"

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m (GRUB2: remove old reference to $UUID (lost with https://wiki.archlinux.org/index.php?title=Creating_Arch_Linux_disk_image&diff=next&oldid=360421))
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=== Generate initramfs for the guest ===
 
=== Generate initramfs for the guest ===
  
{{Accuracy|This step may be unnecessary, for me initramfs generation when pacstrap installed the kernel was succesful. Perhaps just configure the bootloader to use {{ic|/boot/initramfs-linux-fallback.img}} until the initramfs is regenerated from the VM (using the {{ic|autodetect}} hook)?}}
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{{Accuracy|This step may be unnecessary, for me initramfs generation when pacstrap installed the kernel was successful. Perhaps just configure the bootloader to use {{ic|/boot/initramfs-linux-fallback.img}} until the initramfs is regenerated from the VM (using the {{ic|autodetect}} hook)?}}
  
 
As noted earlier, [[initramfs]] generation failed when installing {{Pkg|linux}}.   
 
As noted earlier, [[initramfs]] generation failed when installing {{Pkg|linux}}.   
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=== Other tips ===
 
=== Other tips ===
  
* If you use [[QEMU]], you can specify the guest's kernel and initramfs on the command line.  If you do this, you don't need to install a bootloader on the disk image, nor do you need to install the {{Pkg|linux}} package.
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* If you use [[QEMU]], you can specify the guest's kernel and initramfs on the command line.  If you do this, you do not need to install a bootloader on the disk image, nor do you need to install the {{Pkg|linux}} package.
  
 
* Since you have full control over the guest's hardware, it is not too hard compile a custom [[Kernels|kernel]] for the guest that has all the needed [[Kernel modules|modules]] built in, if you are familiar with configuring the Linux kernel.
 
* Since you have full control over the guest's hardware, it is not too hard compile a custom [[Kernels|kernel]] for the guest that has all the needed [[Kernel modules|modules]] built in, if you are familiar with configuring the Linux kernel.
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* You can make copies of your disk image and run multiple Arch Linux virtual machines at the same time.
 
* You can make copies of your disk image and run multiple Arch Linux virtual machines at the same time.
  
* See [[Install from Existing Linux]] for some more general tips about installing Arch Linux from an existing Linux installation that doesn't necessarily have to be Arch Linux.
+
* See [[Install from Existing Linux]] for some more general tips about installing Arch Linux from an existing Linux installation that does not necessarily have to be Arch Linux.

Revision as of 09:02, 28 April 2015

This page describes how to create a file that contains a disk image of Arch Linux. This disk image can be run in a virtual machine using software such as QEMU, OpenStack, VirtualBox, or VMware, and it can be customized however you want.

Archiso

The official installation media for Arch Linux is a hybrid ISO/disk image, so it can already be booted either as a CD-ROM or as a disk. However, it uses an ISO filesystem, so nothing on it can be changed without re-making the installation media.

You can also create your own live Arch Linux systems using the Archiso tools. This may be what you want, but Archiso is only designed for building live systems that boot from a read-only filesystem.

Install Arch Linux in a disk image using the installation media

Using QEMU, VirtualBox, or other virtualization software, you can install Arch Linux into a disk image by booting the virtual machine from the installation media with the disk image file attached as a virtual hard disk. This is the preferred way to make a virtual disk image containing Arch Linux because other than starting up the virtual machine, the installation will proceed exactly as in the Installation guide.

Install Arch Linux in a disk image without the installation media

It is also possible to create a disk image of Arch Linux directly from software and packages on a host Arch Linux system. This has several advantages:

  • You do not need to have a copy of the installation media.
  • You can include the most up-to-date software packages in the disk image by installing them directly from the host's package manager, and you can do this before you have even booted up the guest for the first time.
  • You can customize the disk image in ways that may not be supported by the official Arch Linux installer.

However, this method is more difficult than using the installation media. In addition, it will not work if your host machine does not have pacman (i.e. is not running Arch Linux).

In these directions, the host system refers to the Arch Linux system you are currently running, while the guest system refers to the Arch Linux system you are creating as a disk image.

Make a file containing the disk image

Create a raw disk image for the virtual machine. In this example, it is made with a size of 1 GiB.

On file systems supporting the fallocate() system call:

$ fallocate -l 1G archlinux.raw

Otherwise, simply use dd:

$ dd of=archlinux.raw bs=1 seek=1G count=0
Note: If you have qemu installed, using qemu-img is another alternative. See QEMU#Creating a hard disk image for details.

Create filesystem(s) on the virtual disk

Use entire disk as one filesystem

If you do not need multiple partitions in your Arch Linux guest, it is easiest to leave the virtual disk unpartitioned and use the whole thing as a filesystem.

To make an ext4 filesystem:

$ mkfs.ext4 -F archlinux.raw

Partitioned disk

Or you can partition the disk. In this simple example, it will be given only one partition, and it will be a bootable primary partition formatted as an ext4 filesystem containing the guest's entire filesystem. There will be no swap partition.

First partition the disk:

Tango-edit-clear.pngThis article or section needs language, wiki syntax or style improvements.Tango-edit-clear.png

Reason: This instruction explains only how and not why, is rather error-prone, and is hard to debug. Why not just link to Partitioning? (Discuss in Talk:Creating Arch Linux disk image#)
$ fdisk archlinux.raw <<< '
$ > o
$ > n
$ > p
$ > 1
$ >
$ >
$ > a
$ > 1
$ > w'

Then make the partitions available as loopback devices. The rest of the instructions will assume that the loopback device created for archlinux.raw is /dev/loop0, but it will be a higher number if you already have set up loop devices.

# losetup -f --show archlinux.raw
# kpartx -a /dev/loop0
Note: kpartx is part of the multipath-toolsAUR package from the AUR. See QEMU#Mounting a partition inside a raw disk image for other ways to mount a partition inside a disk image.

Finally make the needed filesystems on the partitions.

# mkfs.ext4 /dev/mapper/loop0p1

Install packages on the guest's filesystem

Mount the guest's root filesystem on a temporary directory. For convenience, set its path as $TMPDIRj, which will also be used in the following instructions.

# TMPDIR=/full/path/to/temporary/directory

For an unpartitioned disk:

# mount archlinux.raw $TMPDIR

or, for a partitioned disk:

# mount /dev/mapper/loop0p1 $TMPDIR

Install arch-install-scripts and then install the packages you want on the system (like the base group):

# pacstrap $TMPDIR base

Write a fstab file for the guest

Add any mountpoints to the guest's fstab file. In this example, we just need a mountpoint for the guest's root filesystem. You do not have to specify it by UUID, but it is a good idea to do so because it guarantees that the root partition will be found regardless of what type of disk is emulated.

# genfstab -U $TMPDIR >> $TMPDIR/etc/fstab

And then fix up the paths as necessary.

Generate initramfs for the guest

Tango-inaccurate.pngThe factual accuracy of this article or section is disputed.Tango-inaccurate.png

Reason: This step may be unnecessary, for me initramfs generation when pacstrap installed the kernel was successful. Perhaps just configure the bootloader to use /boot/initramfs-linux-fallback.img until the initramfs is regenerated from the VM (using the autodetect hook)? (Discuss in Talk:Creating Arch Linux disk image#)

As noted earlier, initramfs generation failed when installing linux.

You may need to edit $TMPDIR/etc/mkinitcpio.conf to remove the autodetect hook, to stop your host system's hardware configuration from removing essential modules (e.g. those needed to access the root filesystem) from the initramfs of the guest, which is going to be running in a different environment in a virtual machine.

Generate the initramfs for the guest manually by running the following command:

# mkinitcpio -g $TMPDIR/boot/initramfs-linux.img -k $TMPDIR/boot/vmlinuz-linux -c $TMPDIR/etc/mkinitcpio.conf

Install bootloader on the guest

For your bootloader, you can choose Extlinux, GRUB2, or another bootloader.

You may run into some problems when trying to install GRUB2 on a virtual disk. It seems to only work on partition loopback devices created through the device mapper, not those created using the loop module or setting up a loop device with an offset. In addition, GRUB2 may fail to detect your ext4 filesystem for some reason. It is recommended to try Extlinux first, unless you need to use GRUB.

Extlinux

Install Extlinux on the guest's bootable partition:

# extlinux --install $TMPDIR/boot

Install Syslinux's MBR in the guest's MBR (only for partitioned disks):

# dd if=/usr/lib/syslinux/mbr.bin conv=notrunc bs=440 count=1 of=/dev/loop0

Finally, create a configuration file for Extlinux. Replace UUID with the UUID of the guest's root filesystem, which can be obtained by running lsblk -f.

$TMPDIR/boot/extlinux.conf
DEFAULT archlinux
LABEL   archlinux
SAY     Booting Arch Linux
LINUX   /boot/vmlinuz-linux
APPEND  root=/dev/disk/by-uuid/UUID ro
INITRD  /boot/initramfs-linux.img
Note: In order for Extlinux to be booted from the MBR on a partitioned disk, it must be installed on a partition marked as bootable.

GRUB2

In this example, we will install the GRUB2 bootloader on the partitioned disk of the guest.

Install GRUB2. --boot-directory must be set to the /boot directory within the guest's root filesystem, while the given device must be the loopback device corresponding to the guest's entire disk image. Be careful not to overwrite the bootloader of your host system!

# grub-install --boot-directory=$TMPDIR/boot /dev/loop0

Then write a grub.cfg file. Replace UUID, in both places, with the UUID of the guest's root filesystem, which can be obtained by running lsblk -f.

$TMPDIR/boot/grub/grub.cfg
set default="0"
set timeout="3"
insmod msdospart
insmod ext2
set root='(/dev/sda, msdos1)'
search --no-floppy --fs-uuid --set=root UUID
menuentry "Arch Linux" {
   linux /boot/vmlinuz-linux root=/dev/disk/by-uuid/UUID ro
   initrd /boot/initramfs-linux.img
}

Configure cloud-init

This step is optional. Users planning to use the arch linux images on cloud like say to launch in OpenStack would want to perform some early initialization that can be done with cloud-init package. Follow instruction in cloud-init to configure the package.

Cleanup

You should not boot the guest while its filesystem is still mounted; otherwise, the files on it may be corrupted. Unmount the guest's filesystem and get rid of the loopback devices first.

# umount $TMPDIR
# kpartx -d /dev/loop0
# losetup -d /dev/loop0

Boot the guest

Finally, boot the guest Arch Linux using your virtualization software of choice, such as QEMU:

# qemu archlinux.raw

Other tips

  • If you use QEMU, you can specify the guest's kernel and initramfs on the command line. If you do this, you do not need to install a bootloader on the disk image, nor do you need to install the linux package.
  • Since you have full control over the guest's hardware, it is not too hard compile a custom kernel for the guest that has all the needed modules built in, if you are familiar with configuring the Linux kernel.
  • You can make copies of your disk image and run multiple Arch Linux virtual machines at the same time.
  • See Install from Existing Linux for some more general tips about installing Arch Linux from an existing Linux installation that does not necessarily have to be Arch Linux.