I’ve created via zfs a RAIDZ, which is roughly something like a RAID5 in the traditional way. It contained three disks and as time goes by, the resulting array (roughly double the size of a single disk) was nearly full.
I got an identical vendor and size disk like the first three disks and added it to the case. But the RAIDZ-expansion-feature is at the time of this writing still not completed and far from being included in Ubuntu releases.
So I came up with a plan to get a bigger array without loosing any data and without backing up all the data to an external system (I simply didn’t have *that* many spare disks).