&quot;Then I found out it fragments badly, and nobody can figure out how to write a defragmenter. So, uh, keep the FS below 60-80% full apparently.&quot;<p>Confirmed.  Not FUD.<p>Our experience[1] is that things go to hell around 90% and even if you bring it below 90% there is a permanent performance degradation to the pool.  In order to be safe, we try to keep things below 80%, just to be safe.  That&#x27;s probably a bit conservative, though.<p>ZFS needs defrag.  It is <i>not</i> reasonable to give up 3 drives worth of capacity for the parity (raidz3, for instance) <i>and then on top of that</i> set aside another 10-20% as the &quot;angels share&quot;.<p>[1] rsync.net