Diskless Mode

In Diskless mode, the entire operating system with all applications are first loaded into RAM and then only run from there. This is also the method used to boot the Alpine Linux ISO9660 filesystem based read-only installation media. Alpine Linux can be installed and configured so that the system continue to boot in diskless mode, if "disk=none" is specified while running the setup-alpine script.

Diskless mode is extremely fast and can save on unnecessary disk spin-ups, power, and wear and suitable for servers. It is similar to what other linux distributions may call a "frugal" install or boot using a "toram" option.

Diskless installation

Alpine Linux setup script setup-alpine allows custom configurations and package installations to be optionally preserved or made to "persist" across reboots by using Local Backup Utility(lbu). The lbu utility commits and reverts system states by using .apkovl files that are saved to any available writable storage on any media other than the read-only installation media and loaded when booting.

If additional or updated packages have been added to the system, these may also be made available for automatic (re)installation during the boot phase without any (re)downloading, by enabling a local package cache on the writable storage.

Customizable boot device

Customizable boot device allows Alpine Linux to boot and run in Diskless mode from a partition with a writable filesystem on USB-Stick/CompactFlash/SDCard or SSD/NVMe harddisk. Local customizations like .apkovl files and local package cache can also be stored in this customizable boot device.

Using an internal disk for persistent storage

Alpine Linux setup script setup-alpine uses setup-lbu script to save the config and package cache to any available writable filesystem on any media other than the read-only installation media. Due to Bug: #10473 storing local configs and package cache on internal disks requires manual steps, i.e making an entry in /etc/fstab, create mountpoint, and mount the partition before running setup-alpine script.

In this case, boot the target diskless system from the installation media and do not proceed after the boot process stage.
If necessary partition(s) are unavailable, manually create a partition using fdisk. In the below steps, we will use /dev/sdXY as partition number. Adjust the partition identifier as per the output of blkid
- Due to Bug: #11589. The APKOVL loading of diskless setups doesn't work on btrfs and xfs filesystems, or nvme-based devices. So use only ext4 filesystem partitions on classic drives to store diskless mode states.
- mkfs.ext4 creates ext4 filesystem with 64bit feature enabled by default, but extlinux may not be able to boot with that due to Issue #14895. You may need to add "-O ^64bit" to mkfs.ext4 to circumvent this. The below command creates an ext4 partition with disabled journaling, to reduce write operations and allow the disk to spin down after the .apkovl and the packages have been read from the partition during the boot. Install package e2fsprogs using command apk add e2fsprogs, if the command mkfs.ext4 is not available.
# mkfs.ext4 -O ^has_journal,^64bit /dev/sdXY
Due to a bug, the partition can not be mounted to /boot. Configure the /etc/fstab to mount the writable partition to /media/sdXY instead of /boot i.e. conforming to the hot/cold-plug mountpoints.
# mkdir /media/sdXY

# echo "/dev/sdXY /media/sdXY ext4 noatime,ro 0 0" >> /etc/fstab
Mount the partitions listed in /etc/fstab:
# mount -a
Verify that the changes have been applied correctly by looking at the output of mount. Proceed to Finishing diskless installation section without rebooting the computer.

Finishing diskless installation

If setup-alpine has not run before, follow the Installation steps to complete the base configuration. The above preparation should now enable you to choose the partition(sdXY) for saving the local configs and package cache. If asked, there is no need to first unmount the partition, that would only be needed to allow installing on its parent disk. When the Disk & Install option appears, accept the choices as follows:
Which disk(s) would you like to use? (or '?' for help or 'none') [none] Enter where to store configs ('floppy', 'sdXY', 'usb' or 'none') [sdXY]: Enter apk cache directory (or '?' or 'none') [/media/sdXY/cache]:
If setup-alpine has already been run to configure the diskless system, the storage and package cache settings can be configured as follows:
# setup-lbu sdXY # mkdir /media/sdXY/cache # setup-apkcache /media/sdXY/cache
Now the diskless installation can be considered complete. Always remember to commit changes to disk.
# lbu commit

Apkovl

When you boot an alpine installation into diskless mode, it will initialize a fresh new root (/) and any customisations you make (such as changing the password, adding users, etc.) will be lost when you reboot. However, any custom configurations may be preserved or "persist" across reboots by using the Alpine Linux tool named Local Backup Utility(lbu). The initial and possibly read-only installation media can remain the only boot device for the "diskless" system by saving the running state to an .apkovl file, and have these automatically loaded when booting from the boot device.

An Apkovl (APK Overlay file) is a file used for storing local configuration state when running Alpine Linux in Diskless Mode. It stores all configuration files that have changed from the default ones. The filename is <hostname>.apkovl.tar.gz and is stored in a location whose path is defined in /etc/lbu/lbu.conf. The contents from the Apkovl file are overlaid on top of the contents of the apks that are loaded on boot.

The backup tool lbu enables committing and reverting local configuration system state by using .apkovl files that are saved to a backup location and loaded when booting. In Diskless mode, for every change made to the running system to persist across reboot, the command lbu commit must be issued before rebooting the system to update the .apkovl file.

Local Package Cache

When Alpine Linux boots in Diskless Mode, the remote repositories will not be available until after networking has started. That means extra packages newer not in your local boot media would not be available after a reboot, unless they were made to persistent local package cache available on a local, writable, storage device. The local package cache can be stored on the same partition as the .apkovl file.

Loading apkovl from webserver

Alpine's "diskless mode" ISO boot images support boot parameters to load customizations files i.e apkovl from a webserver.

It's possible to load an APKOVL file from a webserver, by supplying a custom url with the APKOVL kernel boot parameter. If you don't have a web server you can run busybox's httpd temporarily to serve an .apkovl - busybox httpd -p 127.0.0.1:80.

Upgrading a Diskless System

When Alping Linux runs in "diskless" or "data" disk mode, Upgrading requires few extra steps in addition to the regular upgrade steps.

If booting a "diskless" system from a read-only device, or as an iso image on writable media, it's not possible to update the boot files (kernel, modules, firmware, ...) that reside on that device.

For customizable boot device, It is possible to update the boot files. However, even then, the kernel, with its modules and firmware files, can still not be updated directly through regular packages updates. Instead, there is the update-kernel script that can generate initfs images and install them together with upgraded kernels.

Before upgrading, install the mkinitfs package as this is required for the generation of the initial filesystem used during boot as follows:
# apk add mkinitfs
Additional initfs features that are missing in the default configuration, like the btrfs filesystem support (at the time of writing, to allow loading .apkovl configs and package cache during boot), may be enabled in the file /etc/mkinitfs/mkinitfs.conf.
Available initfs features may be listed with ls /etc/mkinitfs/features.d

# ls /etc/mkinitfs/features.d # apk add nano # nano /etc/mkinitfs/mkinitfs.conf # lbu commit

Finally update the kernel and its boot environment.

# update-kernel /media/sdXY/boot/

Note: update-kernel run needs at least 8 GB free ram memory to avoid a broken modloop-image. So, in memory constrained devices like Raspberry Pi, use the environment variable TMPDIR to point to a directory on a *nix file system formatted physical device, like a SD-Card or USB-Stick as follows:

# TMPDIR=/media/sdc1/tmp update-kernel /media/mmcblk0p1/boot/

See update-kernel --help for options to manually add additional module or firmware packages.

Kernel options

Documentation about kernel command line options regarding diskless mode will be available after installing the documentation sub-package mkinitfs-doc:

man mkinitfs-bootparam

       If no root= parameter is given, the initramfs will build a live system
       in memory from scratch. This is also called diskless mode.

       When booting in diskless mode, the following options are also
       available:

       alpine_repo=(URL | PATH)
              If set, /etc/apk/repositories will be filled with this. May be a
              comma-separated list of URLs.

       apkovl=(URL | [DEVICE[:FS_TYPE]:]PATH)
              A HTTP, HTTPS or FTP URL to an apkovl.tar.gz file which will be
              retrieved and applied. Can also be a filesystem path, optionally
              prepended with the device name without the /dev/ prefix.

       autodetect_serial=no
              Disable automatic detection and setup of serial console.

       ds=OPTIONS
              Data source for tiny-cloud. If OPTIONS starts with nocloud,
              tiny-cloud will be enabled.

       nokeep_apk_new
              Setup a fresh system, ignore any apkovl.

       pkgs=PACKAGE{,PACKAGE}
              Comma-separated list of packages to be installed.
       ssh_key=(URL | SSH_KEY)
              This setting installs openssh and places the public key given as
              value in /root/.ssh/authorized_keys.  If the value is an HTTP or
              FTP url, its fetches the key(s) from there.

       splash Enable splash screen.

       usbdelay=NUMBER
              Wait NUMBER seconds for USB devices to show up before searching
              for boot media.

       wireguard=INTERFACE;IP_ADDRESS{,IP_ADDRESS,...}[;WG_CONFIG_FILE]
              Set up a wireguard interface named INTERFACE with the addresses
              IP_ADDRESS and use /etc/wireguard/initrd.conf or WG_CONFIG_FILE
              as a classic wg (not wg-quick) config.

       zfs_force=NUMBER
              Enable force importing the root zpool on boot, even if it was
              previously mounted from a different system/OS.