Context

I’ve previously installed Debian and Alpine on my Chromebook and now I want to get experience with PostmarketOS (PMOS).

Environment

I’m running Alpine on an Acer Chromebook 14. I have a 150 GB USB drive to boot from.

Pmbootstrap

PMOS differs from other Linux distributions by providing an installation utility called pmbootstrap rather than ISO image files. As I understand it, PMOS originally targeted mobile devices, and flashing an OS configured for a device directly to the device is more common in that context.

In short, the process below builds and flashes an OS to a USB, which then acts as a host device for installing an OS onto the Chromebook. It’s a bit of a mind-bender, but I like the idea that any device can be used to install the OS on any other device.

I installed it like this:

$ doas apk add pmbootstrap

I initialized it like this:

$ pmbootstrap init

The initialization logic asks several questions about the OS to build, such as architecture (x86_64), desktop manager (gnome), packages to include (gnome-console), etc.

Then I can install the OS like this:

$ pmbootstrap install --disk=/dev/sda --no-recommends --add pmbootstrap,gnome-console

To get the disk identifier, I ran lsblk, installed via util-linux. I didn’t copy down the exact output, but it was something like this:

$ lsblk
NAME         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINTS
sda            8:0    1 116.1G  0 disk 
├─sda1         8:1    1   243M  0 part 
└─sda2         8:2    1 115.8G  0 part
mmcblk0      179:0    0 29.1G  0 disk 
├─mmcblk0p1  179:1    0    1K  0 part /
├─mmcblk0p2  179:2    0    1K  0 part /
└─mmcblk0p3  179:3    0 28.9G  0 part /boot
mmcblk0boot0 179:8    0    4M  1 disk 
mmcblk0boot1 179:16   0    4M  1 disk

The sda disk is my USB drive.

The no-recommends flag alleviated an issue where pmbootstrap ran out of space unpacking firmware for the OS. Since the Chromebook doesn’t need any firmware, I could omit it.

But just passing no-recommends results in a system so minimal it doesn’t even have a shell I can use to install additional packages, so I pass the add flag to include that, and since my intent is to use pmbootstrap to install an OS on the Chromebook, I include pmbootstrap too.

USB Host

Once the install completes, I have an OS on the USB I can use to boot the Chromebook, so the next step is to do just that. I plug the USB into the Chromebook, reboot it, press esc at the Tianocore boot screen, navigate to the boot menu and select the USB. The Chromebook then boots to PMOS!

But as mentioned above, it’s an actual PMOS installation, not an installer, so now I have to repeat the process to install PMOS on the Chromebook. In other words:

$ pmbootstrap install --disk=/dev/mmcblk0 --no-recommends

eMMC

Before I do that, though, I optimize for the eMMC storage. Apparently, eMMC isn’t as good as SSD at wear-leveling, so we generally want to minimize writes.

A few best-practices:

ChromeOS is known to create a lot of partitions, so I clear all the partitions on the disk:

$ doas wipefs --all /dev/mmcblk0

PMOS appears to be aware of eMMC best-practices, so I run pmbootstrap install next.

Then I verify what it did, starting by just dumping the output of lsblk into Gemini. It tells me PMOS set everything up perfectly! It’s using zram for swapping and sets swappiness to 180.

Since the basics are in place, I shutdown at this point, remove the USB and reboot.

The next recommendation is to update the root partition in /etc/fstab to use noatime, so the OS updates file metadata less frequently.

Related, /tmp and /var/log should write to RAM instead of storage. I see /tmp is already doing the right thing, but /var/log isn’t:

$ df -h /tmp
Filesystem                Size      Used Available Use% Mounted on
tmpfs                     1.9G      8.0K      1.9G   0% /tmp
$ df -h /var/log
Filesystem                Size      Used Available Use% Mounted on
/dev/mmcblk0p2           28.3G      4.9G     21.9G  18% /

I drop the output fromls /var/log into Gemini, which says the only concern is journald. I can check the current state:

$ journalctl --disk-usage
Archived and active journals take up 347M in the file system.

I can create a /etc/systemd/journald.conf to route these logs to RAM:

[Journal]
Storage=volatile
RuntimeMaxUse=64M

The final recommendation is to configure Firefox to disable disk caching, enable ram caching and persist session data less frequently. I don’t actually get much value from things like reopening tabs after a crash, so I just disable the session-related features.

Recap

The eMMC optimizations are a rabbit hole, but PMOS sets reasonable defaults and I can just keep an eye on the basic idea of minimizing writes.

I think it’s much more important to acknowledge that I now have PMOS running!