I don't know if the authors are here, but if they are - would you comment on fragmentation and the dangers of growing a filesystem past 95-98% full ?
In the world of ZFS, performance can become significantly degraded with as low as 90% space filled. Further, our experience has been that you can permanently degrade filesystem performance by churning the usage above 95% for any significant amount of time. Which is to say, even if you reduce usage back down to 80%, the zpool maintains poor performance until it is destroyed and recreated.
This is exactly what you would expect to see with a fragmenting filesystem that has no defrag tool.
Unfortunately, creating a defrag tool for ZFS is a very daunting technical hurdle and it appears that nobody is interested in pursuing it.
How does TFS behave ? Does it have, or do you plan for it to have, a defrag utility ?