It only works with 3+ disks. All data needs to be on two disks.
e.g. you have 3 100GB drives, total capacity in raid 1 is 150GB.
If you replace a broken one with a 200GB one, the total capacity will be increased to 200GB.
e.g. you have 3 100GB drives, total capacity in raid 1 is 150GB.
If you replace a broken one with a 200GB one, the total capacity will be increased to 200GB.
You can then extend the pool by adding more such partition-based vdevs as you replace disks, just add the new partitions and add new vdevs.
So if you have a 1TB disk, a 2TB disk and a 4 TB disk, you could have mirrors (d1p1,d3p1), (d2p1,d3p2), (d2p2,d3p3) with a total of 3TB mirrored, and 1 TB available. If you swap the 1TB for a 2TB disk, partition it, replacing the old d1p1 partition with the new and resilver, and once that's done you can add the mirror (d1p2,d3p4) and get the full 4TB redundant storage.
Not a great solution though, as it requires a lot of manual work, and especially write performance will suffer because ZFS will treat the vdevs as being separate and issue IOs in parallel to them, overloading the underlying devices.