How to switch your snap to use LZO compression

[ijohnson]

Recently we enabled using LZO as a compression format for your snap in order to get better decompression performance for i.e. desktop application snaps. Specifically using LZO compression instead of the default XZ compression will help speedup the time it takes your snap to launch the very first time on a user’s system, at the cost of a slightly larger snap file. You can read more details about this in this blog post.

If you would like to switch to using LZO compression with your snap, all you need to do is to add the compression setting to the snapcraft.yaml like so:

name: test-snapcraft-lzo
base: core18
version: "0.1"
summary: Test LZO snap
description: Test LZO snap
grade: stable
confinement: strict


# this line enables LZO compression for the snap
compression: lzo


parts:
  my-part:
    plugin: nil

apps:
  lzo-things:
    command: bin/something

And then re-build the snap through whatever normal release mechanisms you use to build the snap and release it, and voila your snap is now using LZO compression.

[Lin-Buo-Ren]

Is there a minimum requirement of the snapd version to support this compression format?

[James-Carroll]

@Lin-Buo-Ren (Artificially increasing the length of this post because 20 characters bla bla bla)

@ijohnson

The TLDR is that snapd doesn’t actually care about what format of compression the snap is in - only the store and kernel really cares.

In reality, snapd only cares about the compression format of the snap when it is creating the snap file, that is at build time with snap pack. The rest of the time, snapd treats it as a binary blob downloaded from the store, and instructs systemd to mount the .snap file, and systemd itself relies on the kernel to mount the snap. The only exception to this (and was in fact one of the bugs that we had to fix for 2.47.1 before LZO was allowed to be readily available), is inside container environments where the container is not allowed to mount things via the kernel and instead uses a FUSE (filesystem in userspace) implementation - that FUSE implementation must support mounting LZO compressed squashfs files

[Lin-Buo-Ren]

Thanks for your pointers!