[ironic] [tripleo] RFC: lzma vs gzip for compressing IPA initramfs

Jeremy Stanley fungi at yuggoth.org
Sat Nov 28 14:12:44 UTC 2020


On 2020-11-28 13:32:09 +0100 (+0100), Dmitry Tantsur wrote:
> Is it wildly supported for initramfs compression? I can, of
> course, try, but maybe you know.
[...]

It looks like you need your kernel built with with:

    CONFIG_HAVE_KERNEL_XZ=y
    CONFIG_KERNEL_XZ=y
    CONFIG_RD_XZ=y
    CONFIG_XZ_DEC=y
    CONFIG_XZ_DEC_X86=y (or relevant CPU architecture)
    CONFIG_DECOMPRESS_XZ=y

The standard Linux 5.9 kernel on my Debian systems has the above
enabled by default, not sure about other distros or older kernels
though CONFIG_KERNEL_XZ has been included as far back as Linux 2.38:

    https://cateee.net/lkddb/web-lkddb/KERNEL_XZ.html

Apparently you may need to pass --check=crc32 and (possibly
--lzma2=dict=512KiB as well) to xz when compressing your initrd, to
accommodate the kernel's decompressor implementation:

    https://github.com/linuxboot/book/blob/master/coreboot.u-root.systemboot/README.md#building-u-root
    https://www.linuxquestions.org/questions/slackware-installation-40/booting-with-xz-compressed-initrd-4175599273/

-- 
Jeremy Stanley
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