Given that other file systems (notably: xfs) support reflinks these days, let's
extend the file system snapshotting logic to fall back to plan copies or
reflinks when full btrfs subvolume snapshots are not available.
This essentially makes "systemd-nspawn --ephemeral" and "systemd-nspawn
--template=" available on non-btrfs subvolumes. Of course, both operations will
still be slower on non-btrfs than on btrfs (simply because reflinking each file
individually in a directory tree is still slower than doing this in one step
for a whole subvolume), but it's probably good enough for many cases, and we
should provide the users with the tools, they have to figure out what's good
for them.
Note that "machinectl clone" already had a fallback like this in place, this
patch generalizes this, and adds similar support to our other cases.
Usually, we place the #pragma once before the copyright blurb in header files,
but in a few cases we didn't. Move those around, so that we do the same thing
everywhere.
It's not a good idea to create subvolumes for parts of the OS tree (such
as /home, or /var) if the root directory is not a subvolume too. We
shouldn't assume control of "heavier" objects such as subvolumes, if the
originating object (the root directory) is a "light-weight" object, i.e.
a plain directory.
Effectively this means that chroot() environments that are run on a
plain directory do not have to deal with problems around systemd
creating subvolumes that cannot be removed with a simple "rm" anymore.
However, if the chroot manager creates a proper subvolume for such an
environment it will also get further subvolumes placed in there, under
the assumption that the manager understands the concept of subvolumes in
that case.
With this change we understand more than just leaf quota groups for
btrfs file systems. Specifically:
- When we create a subvolume we can now optionally add the new subvolume
to all qgroups its parent subvolume was member of too. Alternatively
it is also possible to insert an intermediary quota group between the
parent's qgroups and the subvolume's leaf qgroup, which is useful for
a concept of "subtree" qgroups, that contain a subvolume and all its
children.
- The remove logic for subvolumes has been updated to optionally remove
any leaf qgroups or "subtree" qgroups, following the logic above.
- The snapshot logic for subvolumes has been updated to replicate the
original qgroup setup of the source, if it follows the "subtree"
design described above. It will not cover qgroup setups that introduce
arbitrary qgroups, especially those orthogonal to the subvolume
hierarchy.
This also tries to be more graceful when setting up /var/lib/machines as
btrfs. For example, if mkfs.btrfs is missing we don't even try to set it
up as loopback device.
Fixes#1559Fixes#1129
We already had a safety check in place that we don't end up descending
to the original subvolume again, but we also should avoid descending in
the newly created one.
This is particularly important if we make a snapshot below its source,
like we do in "systemd-nspawn --ephemeral -D /".
Closes https://bugs.freedesktop.org/show_bug.cgi?id=90803
