Dev at Ubuntu Dimitri John Ledkov who has previously been involved in this effort commented on the Ubuntu mailing list with this update:
Zstd patches have not made it into the upstream kernel yet.
As used by mkinitramfs: – lz4 is faster to compress than gzip – lz4 is blazingly fast to decompress – lzma is dog slow to compress and decompress, but is tiny – lz4 size weight over gzip is marginal (14%) but imho worth the improved boot time & initrd creation time – xz is potentially even slower and even smaller than lzma
In places where size is an absolute premium (tiny embedded iot devices) and performance is irrelevant, xz or lzma should be used.
In all other places, our performance profile is in favor of lz4.
Imho that includes the kernel image itself, thus we should consider switching: – initramfs tools to default to lz4 – livecd-rootfs to default to lz4 – kernels to compress kernel image with lz4 – grub to include lz4 support
I shall proceed with changing the defaults on the above to improve our responsiveness experience on installer, cloud, core and classic devices. If our firstboot & subsequent boot speed degrades or disk space becomes a concern, we can look into tweaking these changes further.
So, we can hope for this in Ubuntu 19.10 for testing/updates before (hopefully) being released in Ubuntu 20.04 LTS.