OpenZFS on Debian: zpool mirroring does not work on Debian, systemd hanging on reading zfs cachefile, etc

[kernschmelze]

I am trying out to use ZFS on Debian.
It turns out to be seriously deficient in comparison to ZFS on FreeBSD.

In particular, it is impossible to split pools.
This always fails with the message that only mirrors can be split, even when explicitly specifying the mirror to split.

So I tried to detach the drives that needed to be removed from the pool.
This always failed because of "device busy".
So it seems that maybe issue #807 is still open, even if the according issue has been closed.
So I tried as last resort to shut down and just remove the drives that need to be un-pooled.
After having done that, it was no longer able to import the 4-way mirror that was degraded to 2 drives, as ZFS on Linux now complained that it were unable to find the pool even though zdb recognizes it.

So I would like to know if ZFS mirroring is functional on Linux at all.

I also need to mention that I had to write-protect and put the immutable attribute on the zpool cache file in /etc/zfs/.
If this is not being done, just putting in a drive with a ZFS pool and removing that again is a surefire way to brick a ZFS on Linux installation.
Thus I do not understand why the Debian ZFS installation guide instructs users to use zfs cachefiles, even if this is deprecated.

On FreeBSD all this works just as expected.

So why does it not work on Linux?

Right now the question how to split up a ZFS mirrored pool or to detach drives on Linux is the most pressing issue for me - any idea how to achieve this?

[kernschmelze]

Addeddum:
To be able to make reproductible DebianOnZFS installations, I wrote an interactive installer for Debian
https://github.com/kernschmelze/DebianOnZFSInstaller