a year ago we changed the default compression and level for the
initramfs to zstd -1. This fixed the very slow creation times on
development boards (see bug #1958148), but that leads to bigger
initramfs sizes that triggered other bugs (like bug #1842320).
Big initramfs sizes can also fill up small sized /boot partitions easily
(grooming the 850 initramfs-tools bugs revealed several such reports).
Using xz -9 would give very good compression, but it takes very long
(especially on slow development boards) and a lot of memory (good luck
on Raspberry Pis with small memory like Pi Zeros).
I propose following approach to address the drawback: Create cpio
archives (compressed with xz -9) for the kernel modules and firmware
files when building the kernel/firmware Debian package. Then ship those
cpio archives in the package (or in a separate binary package). Then the
CPU load it put on the builders. The cpio archives would contain the
modules for MODULES=most.
mkinitramfs will then look for those cpio archives and uses those in
case they are present. Such a initramfs would look like this:
* AMD/Intel microcode cpio archive (on amd64)
* main cpio archive compressed with zstd -1
* kernel modules from the Debian package compressed with xz -9
* firmware files from the Debian package compressed with xz -9
After working on initramfs-tools as part my day job, my fingers were
itching and I had to create a quick and dirty draft in my free night
time. You can find the result of the last two hours in [1]. This draft
has a mkinitramfs-kernel script that creates a cpio archive containing
the kernel modules and firmware (that needs to be split later on).
The lunar test result on my AMD Ryzen 7 5700G look promising: Building
6.2.0-24-generic-modules-most.cpio.xz takes around 90 seconds and is
54.9 MiB in size. Creating the initramfs speeds up from around 8.7
seconds to 3.5 seconds (saves 60 %). The size reduces from 133.1 MiB to
80.7 MiB (saves 39.4 %). So the boot needs 52.4 MiB less, but
/lib/modules need 54.9 MiB for the cpio archive.
The drawback is that building the kernel would take longer, the package
takes more space on the archive and mirrors, and downloading them could
take longer on slow connections.
Implementing my proposal would be relative easy for initramfs-tools, but
would mean some work for the kernel team.
What do you think?
--
Benjamin Drung
Debian & Ubuntu Developer
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