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 the method already used to boot the Alpine Linux iso installation images. Alpine Linux can be installed and configured so that the system continue to boot like this if "disk=none" is specified while running the setup-alpine script.

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

Custom configurations may be preserved or "persist" across reboots by using the Alpine Linux tool named local backup Utility(lbu).

Apkovl

Apkovl 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 contents are overlaid on top of the contents of the apks that are loaded on boot. The filename is <hostname>.apkovl.tar.gz and is stored in a location whose path is defined in /etc/lbu/lbu.conf.

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. For every change made to the running Diskless 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, the remote repositories will not be available until after networking has started. That means packages newer than on your local boot media would not be available after a reboot, unless they were made to persistent, by having a local package cache available on a local, writable, storage device.

If additional or updated packages have been added to the system, these may also be made available for automatic (re)installation and copied into RAM during the boot phase without any (re)downloading, by enabling a local package cache on a writable storage. The local package cache can be stored on the same partition as the .apkovl file.

Installation

During Installation, Alpine Linux Setup script Setup-alpine uses setup-lbu script to configure a "local backup" location to save the initial local configuration state as an .apkovl file using lbu tool. If subsequent changes are made to the running system, the command lbu commit must be used to update the .apkovl file.

Boot the diskless system from ISO and follow the Installation steps to complete the base configuration. Now the pre-setup of "diskless" Alpine Linux system is considered complete.

If necessary partition(s) are unavailable, manually create a partition. We will use /dev/sdXY as the name of internal partition to save and load the .apkovl file.

Once a writable partition i.e /dev/sdXY is available, setup-alpine can be told to store the configs and the package cache on that writable partition. Due to bug: #10473 storing local configs and the package cache on internal disks requires manual steps as listed under Apkovl on system partitions before running setup-alpine script.

Later, another directory on that same partition or another available partition may also be mounted as /home, or for example, for selected important applications to keep their run-time and user data on it.

Apkovl on internal disk

Alpine's "diskless mode" ISO image based boot device can load an .apkovl volume from writable partitions on internal disks i.e system partitions. It is therefore possible to save customized running states to a .apkovl file on a writable partition, and have these automatically loaded when booting the ISO image.

To save customized running states to a .apkovl file on a writable partition on internal disks, and have these automatically loaded when booting the boot device i.e ISO image, the following steps are needed to manually prepare a specific /etc/fstab entry, and save the configuration, as follows:

Boot the diskless system from ISO and follow the Installation steps to complete the base configuration. Now the pre-setup of "diskless" Alpine Linux system is considered complete.
If necessary partition(s) are unavailable, manually create a partition. For the steps, we will use /dev/sdXY
1. 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.
2. 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.
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
Use mount -a to load the partitions listed in /etc/fstab. Look at the output to verify that the changes have been applied correctly.
If setup-alpine has not run before, it may be used and should now enable you to choose the partition for saving the local configs and package cache.
If setup-alpine has already been used to configure the diskless system, the storage settings may be modified directly with
setup-lbu sdXY
Local package cache can be enabled as follows:
mkdir /media/sdXY/cache

setup-apkcache /media/sdXY/cache
If the partition is large enough, it can be useful to edit lbu.conf to uncomment and set BACKUP_LIMIT=3. For example, to allow reverting to a previous, working state if needed.
apk add nano

nano /etc/lbu/lbu.conf
Finally, generate the first .apkovl file containing all the previous changes by executing lbu commit, so the customizations that were just made will persist a reboot.
lbu commit
From now on, whenever packages are installed or newly configured, and the changes should be kept, execute lbu commit.

Customizable boot device

Alpine Linux running Diskless mode can also boot from a partition with a writable filesystem on USB-Stick/CompactFlash/SDCard or SSD/NVMe harddisk. This boot device is known as customizable boot device to differentiate against iso9660 filesystem based read only Installation Media based boot device. Local customizations like apkovl files and cached packages can be stored in this customizable boot device, which also allows to also upgrade the kernel with its modules and firmware with the update-kernel script.

For the newly configured local "diskless" system, it is possible to copy the boot system from a read-only installation media to a writable vfat partition (e.g. /dev/sdXY) with setup-bootable script or manually to any writable partition and create a customizable boot device.

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.