As a few others mentioned, the 80% issue is one. We try and keep all machines below that so performance doesn't degrade. With the number of drives we have (200+

As a few others mentioned, the 80% issue is one. We try and keep all machines below that so performance doesn't degrade. With the number of drives we have (200+ PB total space) we also run into issues where there are "holes" in the ZFS array after replacing a disk. We've always had enough redundancy work around any issues we find. Overall, I cannot speak highly enough of ZFS!