I actually tested this, and could not measure any performance hit at all until I hit 95% full.

It sped right back up again when I deleted some stuff.

I wouldn't worry about it.

Interesting test. Thanks for finding this out.

You can ZFS send back and forth between pools or to a new dataset to defrag/fix that, so it's not "permanent" as in can't ever be fixed. It also depends on workload, write once/read many is different from VM disk files.

Btrfs can "rebalance" in place. This has had bugs in the past, but seems safe now.

On the side of ZFS, you can take one volume of a mirrorset, move it to another system and mount it in degraded mode, and copy a bunch of new files to it. When you return the updated drive to the full set and mount it, it will resilver the older drive with the newer content automatically.

That's a killer feature. With btrfs, you have to rebalance to mirror the new files, which takes a long time as every block must be rewritten between the mirrors. ZFS knows how to move just the new files in a resilver.