1. Author dislikes ZFS because you can't grow and shrink a pool however you want.
2. Author likes BTRFS because it implements more flexible grow/shrink, but finds that in other ways it's unstable, and is unhappy that BTRFS doesn't let you have arbitrary numbers of parity drives.
I believe that this was on the future timeline for ZFS. It required something like the ability to rewrite metadata or something.
The problem is that nobody really cares about this outside of a very few individual users. Anybody enterprise just buys more disks or systems. Anybody actually living in the cloud has to deal with entire systems/disks/etc. falling over so ZFS isn't sufficiently distributed/fault tolerant.
So, you have to be an individual user, using ZFS, in a multiple drive configuration to care. That's a really narrow subset of people, and the developers probably give that feature the time they think it deserves (ie. none).
IIRC, there were some other nice things that this enabled as well.
Would definitely prefer BTRFS to be stable over having additional features...
If you remove that clause, I would think there are way more people in that situation. That still wouldn't imply that those who write ZFS have to care, though. There's nothing wrong with providing a great solution for a selected subset that isn't suitable for the majority. In ZFS's case, that certainly applies. For its target audience, the ability to shrink pools is nice to have, and nowhere to minimum viable product.
That argument applies to btrfs, too, by he way.cthere are way more users for whom something like Google drive is a better option than running their own btrfs system.cthat doesn't mean there is no reason to develop btrfs.
However it's clear from the docs that you can grow zfs pools by adding new vdevs. Shrinking a pool is not possible.
In practice I haven't seen the inability to shrink pools as an issue. I simply do not create pools from entire disks and use partitions instead. For example each drive is split into 5 partitions. And if I need to grow a pool I add another partition across all my disks to the pool as a new vdev.
This also allows me to have multiple pools on each system that have different Raid-z levels based on different app reqs.
http://blog.delphix.com/alex/2015/01/15/openzfs-device-remov...
To me, this doesn't seem like a big deal.