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When dealing with copying COW images, we have to make a tradeoff:
- Either we don't touch the NOCOW bit on the copied file COW and get
an instant copy because we're able to reflink, but we might get
reduced performance if the source file was COW as COW files and lots
of random writes don't play well together.
- Or we force NOCOW for the copied file, which means we have to do a
full copy as reflinking from COW files to NOCOW files or vice versa
is not supported.
In exec-invoke.c, we've opted for the first option. In nspawn.c and
discover-image.c, we've opted for the second option.
In nspawn, this applies to the --ephemeral option to make ephemeral
copies. In discover-image.c, this applies to cloning images into
/var/lib/machines. Both these features might be used to run many
machines of the same original image. We really don't want to force
a full copy onto users in these scenarios when they're expecting
reflink behavior, leading to them running out of disk space. Instead,
degraded performance in their machines is a much less severe issue,
which they will discover on their own if it affects them, at which
point they can make their original image NOCOW at which point they'll
get both the reflinks and better performance.
Given the above reasoning, let's switch nspawn.c and discover-image.c
to use COPY_NOCOW_AFTER as well instead of enabling NOCOW upfront and
forcing a copy if the original source image is COW.
Unless otherwise requested, if we're going to copy a nocow file, make the
target file nocow as well.
Aside from keeping the performance characteristics of the cow or nocow file
intact, reflinking also only works from cow to cow or nocow to nocow files.
Reflinking from cow to nocow or nocow to cow files does not work and can
easily lead to unexpected copies for users, so by keeping the nocow bit
intact across copies by default we also make sure reflinks always work.
Instead of parsing the human readable output of apt-cache, let's
use apt patterns to figure out the dependencies.
We also filter out virtual packages as apt will fail and say we need
to install an implementation of the virtual package even if a package
that provides the virtual package is already installed.
Now that mkfs.btrfs is adding support for compressing the generated
filesystem (https://github.com/kdave/btrfs-progs/pull/882), let's
add general support for specifying the compression algorithm and
compression level to use.
We opt to not parse the specified compression algorithm and instead
pass it on as is to the mkfs tool. This has a few benefits:
- We support every compression algorithm supported by every tool
automatically.
- Users don't need to modify systemd-repart if a mkfs tool learns a
new compression algorithm in the future
- We don't need to maintain a bunch of tables for filesystem to map
from our generic compression algorithm enum to the filesystem specific
names.
We don't add support for btrfs just yet until the corresponding PR
in btrfs-progs is merged.
