We were discussing the recommended ZFS storage pool capacity recently so it seems like a good time to review this information externally as well.
The recommended pool capacity was changed from 80% to 90% starting in the Oracle Solaris 11 and Oracle Solaris 10 1/13 releases, which is reflected in ZFS file system version 5. However, there are some nuances to review.
Oracle Solaris ZFS is designed with a redirect on write architecture so it needs some headroom for finding space to write new data. This is the reason why we provide a general recommendation of keeping ZFS storage pools below 90% capacity, but the reality is that the percentage where performance is impacted depends greatly on the workload:
Other storage vendors with ROW-based file systems often report reduced overall storage capacity in order to maintain this headroom (and hide the need for the headroom). Our position is that it is up to you to determine how your pool space is utilized.