Array free space requirement

Solution 1:

Yes, it's still the convention, and yes, it holds true even as you scale. With ZFS, in fact, you really don't want to get to the 75% mark in your zpool too often. Fragmentation, snapshots and general performance tend to be impacted.

If building anew, don't start with anything more than 40% utilization and be sure to plan for growth.

Solution 2:

It really depends on your usage, though. Big files that are written once, and then only read? Doesn't fragment, can fill up closer to capacity.

Traditional UNIX home directories with lots of small files, some rewriting, lots of snapshots. Maybe even snapshot retention that keeps lots of recent snapshots, but removes some of them while keeping some older ones. Terrible fragmentation, pool really needs free space to perform.

The only way to know for sure is to test and monitor the performance.