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 into with a "toram" option.

Custom configurations may be preserved or "persist" across reboots by using the Alpine local backup tool lbu. It enables committing and reverting local configuration system state by using .apkovl files that are saved to a "local backup" location i.e a writable partition and loaded when booting.

Installation

Setup-alpine script can store the configs on a writable partition using lbu. If a writable partition is available, setup-alpine can be told to store the configs and the package cache on that writable partition. Use the setup-lbu script to configure a "local backup" location for the diskless system, and lbu commit to then save the local configuration state as an .apkovl file.

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. 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.

As per Bug: #10473 Storing local configs and the package cache on internal disks requires manual steps before running setup-alpine script.

Saving and loading ISO image customizations

To use partitions on internal disks, however, you need to prepare a specific /etc/fstab entry manually, 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 examples we will use /dev/sdXY
  
  • Be aware of this Bug: #11589. The APKOVL loading of diskless setups doesn't work on btrfs and xfs filesystems, or nvme-based devices => So, for the moment, use only ext4 filesystem partitions on classic drives to store diskless mode states.
  • mkfs.ext4 creates ext4 fs 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. ("^" means "not").

mkfs.ext4 -O ^has_journal,^64bit /dev/sdXY

• The above 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.
• 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

• Mounting read-only is possible, because the lbu tools can temporarily remount it writable for their operation. 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 asked, there is no need to first unmount the partition, that would only be needed to allow installing on its parent disk.)
  
• 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.

From then on, whenever packages are installed or newly configured, and the changes should be kept, execute lbu commit again.

Customizable boot device

Main article: Create a Bootable Device

In the above case, the initial read-only installation media may remain the only boot device for the newly configured local "diskless" system. It is possible to copy the boot system to a partition (e.g. /dev/sdXY) with setup-bootable script and create a customizable boot device.

Other Topics

Todo: Need to document the steps

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.

See Also

• Alpine Local backup Utility - lbu'
• Local package cache
• Diskless PXE Boot
• How to make a custom ISO image with mkimage
• QEMU Diskless example
• Include special files section - To include custom files outside of /etc in .apkovl file.