How to switch your snap to use LZO compression

James-Carroll · July 24, 2021, 3:29pm

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

When to contact upstreams to use LZO compression?

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