Thanks for your response _Gea - I think you are entirely correct about the newer version of ZFS having less leniency with remaining free space calculations. The remaining space was approximately 1% - speeds were actually pretty good (2 vdevs of 10 drives each in RAIDZ2). I never had any performance problems, which may surprise some people. I had to free up an additional 1 TB GB or so and everything was fine. I actually booted up my OI151a VM so I could monitor the amount of space freed.