I hacked together a proof of concept implementation which uses
`BTRFS_IOC_ENCODED_WRITE` to write a zstd-compressed cpio payload directly to
disk as-is, from a carefully aligned rpm. Compressed extents are then reflinked
to the installation path.
presentation:
https://www.youtube.com/live/qNGw8IKaqc0?si=vrLkk8Bi9Odfqm4Z&t=10325
rpm-rs based source:
https://github.com/ddiss/rpm/tree/poc-btrfs-zstd-encoded-io-reflink-extract
The rpm payload format isn't modified, although there's a slight "bending" of
the cpio/newc spec to use the filename field for padding. zstandard frames
making up the compressed rpm payload explicitly carry both compressed and
uncompressed lengths, to allow detection of filesystem-supported I/O sizes.
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