Context

I’m using an Acer Chromebook 14, running PostmarketOS (pmOS) v26, as a host for installing pmOS on a Pixel 3a. My main concern is pmOS’ pmbootstrap builds the OS for a target device before installing it and this Chromebook only has 30GB of storage and the storage is eMMC which is relatively fragile. This post documents the set up.

Zram

Prevent OOM (Out Of Memory) crashes during heavy package compilations by configuring ZRAM to allocate around 150-200% of total RAM size as compressed swap space:

$ sudo apk add postmarketos-zram-systemd
...
$ sudo vi /etc/systemd/zram-generator.conf
$ sudo systemctl daemon-reload
$ sudo systemctl start /dev/zram0
$ zramctl
NAME       ALGORITHM DISKSIZE  DATA  COMPR  TOTAL STREAMS MOUNTPOINT
/dev/zram0 zstd          5.7G    1G 183.1M 190.8M         [SWAP]

I see /dev/zram0 mapped to zstd compression with the newly defined disk size limit ✔️

Augmented Storage

Next, I’ll protect the host eMMC write endurance and provide the ~20GB+ needed for pmbootstrap chroots by using an external drive for the pmbootstrap work space.

I have a 120GB USB disk I’ve been using as a boot device when installing pmOS on my Chromebook. Now I want to use it as the workspace for pmbootstrap, to take storage pressure off the Chromebook.

I see the identifier is /dev/sda and one of the partitions is mounted:

$ lsblk -f
NAME FSTYPE FSVER LABEL UUID FSAVAIL FSUSE% MOUNTPOINTS
sda                                                                              
├─sda1                                                                           
└─sda2                       101.4G  5%     /run/media/erik/pmOS_root
...

Unmount it:

sudo umount /dev/sda*

NOTE: the wipefs command below is destructive, so verify your drive identifier with lsblk before executing wipefs.

Wipe the disk clean:

$ sudo wipefs -a /dev/sda
...       
/dev/sda: 5 bytes were erased at offset 0x00008001 (iso9660): 43 44 30 30 31
/dev/sda: 8 bytes were erased at offset 0x00000200 (gpt): 45 46 49 20 50 41 52 54
/dev/sda: 8 bytes were erased at offset 0x1d041ffe00 (gpt): 45 46 49 20 50 41 52 54
/dev/sda: 2 bytes were erased at offset 0x000001fe (PMBR): 55 aa
/dev/sda: calling ioctl to re-read partition table: No error information

Use fdisk to create a standard partition table and a new partition for the ext4 filesystem:

  1. Open the disk utility:
  2. Type g and press Enter to create a new, empty GPT partition table.
  3. Type n and press Enter to create a new partition. Press Enter for all subsequent prompts to accept the default sizes (this allocates the whole drive).
  4. Type w and press Enter to write the changes and exit.
$ sudo fdisk /dev/sda
...
Command (m for help): g
Created a new GPT disklabel (GUID: C4CF7C96-639D-48CC-B88A-4205042F455B).

Command (m for help): n
Partition number (1-128, default 1): 
First sector (2048-243404766, default 2048): 
Last sector, +/-sectors or +/-size{K,M,G,T,P} (2048-243404766, default 243402751): 

Created a new partition 1 of type 'Linux filesystem' and of size 116.1 GiB.

Partition #1 contains a vfat signature.
Do you want to remove the signature? [Y]es/[N]o: Y

The signature will be removed by a write command.

Command (m for help): w
The partition table has been altered.
Calling ioctl() to re-read partition table.
Syncing disks.

The new partition shows up as sda1:

$ lsblk
NAME         MAJ:MIN RM   SIZE RO TYPE MOUNTPOINTS
sda            8:0    1 116.1G  0 disk 
└─sda1         8:1    1 116.1G  0 part 
...

Run the format command:

sudo mkfs.ext4 /dev/sda1
...
mke2fs 1.47.4 (6-Mar-2025)
Creating filesystem with 30425088 4k blocks and 7610368 inodes
Filesystem UUID: 95251614-a755-4a93-8590-48bd34935ea1
Superblock backups stored on blocks: 
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208, 
        4096000, 7962624, 11239424, 20480000, 23887872

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (131072 blocks): done
Writing superblocks and filesystem accounting information: done

Configure Pmbootstrap To Use External Storage

Now, I can mount the drive to a target directory:

$ sudo mkdir -p /mnt/pmos_usb
$ sudo mount /dev/sda1 /mnt/pmos_usb

TIP: create an fstab entry to automatically remount on boot.

Set directory ownership to my unprivileged user so pmbootstrap can write to it without root:

$ sudo chown -R $USER:$USER /mnt/pmos_usb

Initialize pmbootstrap with USB as storage location:

$ pmbootstrap init                                                                                                                                           
[14:35:40] Location of the 'work' path. Multiple chroots (native, device arch, device rootfs) will be created in there.                                                  
[14:35:40] Work path [/home/erik/.local/var/pmbootstrap]: /mnt/pmos_usb/pmbootstrap                                                                                      
[14:35:45] Location of the 'pmaports' path, containing package definitions.                                                                                       
[14:35:45] pmaports path [/mnt/pmos_usb/pmbootstrap/cache_git/pmaports]:                                                                                          
[14:36:06] Setting up the native chroot and cloning the package build recipes (pmaports)
...
[14:36:50] Choose the postmarketOS release channel.
[14:36:50] Available (14):
[14:36:50] * edge: Rolling release / Most devices / Occasional breakage: https://postmarketos.org/edge             
[14:36:50] * v26.06: Latest release / Recommended for best stability
[14:36:50] * v25.12: Old release (supported until 2026-07-31)   
[14:36:50] Channel [edge]: v26.06
...
[14:37:11] Choose your target device vendor (either an existing one, or a new one for porting).
[14:37:11] Available vendors (75): acer ... google ... zte
[14:37:11] Vendor [qemu]: google                                                 

[14:37:17] Devices are categorised as follows, from best to worst:               ...
* Community: often mostly usable, but may lack important functionality.          ...
Available devices by codename (25): asurada (community) ... sargo (community) ... x64cros (community)
[14:37:17] Device codename: sargo

[14:37:47] Username [user]: erik 

[14:37:54] Available providers for postmarketos-base-ui-audio-backend (2):
[14:37:54] * pulseaudio: Use pulseaudio as the audio backend. (default)
... 
[14:37:54] Provider [default]:

[14:38:02] Available providers for postmarketos-base-ui-wifi (2):
[14:38:02] * wpa_supplicant: Use wpa_supplicant as the WiFi backend. (default)
...
[14:38:02] Provider [default]:

[14:38:04] Available providers for postmarketos-usb-moded-default-profile (2): 
[14:38:04] * developer: Make 'developer mode' the default usb-moded profile (always enables usb networking) (default)
...
[14:38:04] Provider [default]:

[14:38:09] Available user interfaces (27):
[14:38:09] * phosh: (Wayland) Mobile UI using GNOME components and apps          ...                    
[14:38:09] User interface [console]: phosh 

[14:38:18] Based on your UI selection, 'default' will result in choosing systemd.
[14:38:18] Which service manager should be used? (default/openrc/systemd) [default]:

[14:38:40] Additional options: extra free space: 0 MB, boot partition size: 512 MB, parallel jobs: 4, ccache per arch: 5G, sudo timer: False, mirror: http://mirror.postmarketos.org/postmarketos/ 
[14:38:40] Change them? (y/n) [n]:

[14:38:51] Additional packages that will be installed to rootfs. Specify them in a comma separated list (e.g.: vim,file) or "none"
[14:38:51] Extra packages [none]:

[14:39:01] Your host timezone: America/Los_Angeles
[14:39:01] Use this timezone instead of GMT? (y/n) [y]:

[14:39:07] Choose your preferred locale, like e.g. en_US. Only UTF-8 is supported, it gets appended automatically. Use tab-completion if needed.
[14:39:07] Locale [en_US]:

[14:39:15] Device hostname (short form, e.g. 'foo') [google-sargo]: pixel3a

[14:39:53] SSH public keys found (1):
...
[14:39:53] Would you like to copy these public keys to the device? (y/n) [n]: 

[14:40:01] After pmaports are changed, the binary packages may be outdated. If you want to install postmarketOS without changes, reply 'n' for a faster installation.    
[14:40:01] Build outdated packages during 'pmbootstrap install'? (y/n) [y]:

[14:40:05] DONE!

Limit Build Concurrency

Prevent CPU starvation on a low-power processor by telling pmbootstrap to limit parallel jobs when compiling larger components:

$ pmbootstrap config jobs 2
jobs = 2