You really shouldn't run non-CoW file systems above 90%, to include UFS and ext
What I believe, and what I think others have also concluded, is that it shouldn't be fatal. That is, when the dust has settled and you trim down usage and have a decent maintenance outage, you should be able to defrag the filesystem and get back to normal.
That's not possible with ZFS because there is no defrag utility ... and I have had it explained to me in other HN threads (although not convincingly) that it might not be possible to build a proper defrag utility.
Doing so requires that you have enough zfs filesystems in your pool (or enough independent pools) that you have the free space to temporarily have two copies of the filesystem.
Yes, and that is why I did not mention recreating the pool as a solution. If your pool is big enough or expensive enough, that's still "fatal".
(2) If your pool is big enough/expensive enough, surely you've also budgeted for backups.
(2) This has nothing to do with backups or data security in any way - it's about data availability (given a specific performance requirement).
You're not going to restore your backups to an unusable pool - you're going to build or buy a new pool and that's not something people expect to have to do just because they hit 90% and churned on it for a while.
I agree it's not ideal to have filesystems do this, but it also simplifies a lot of engineering. And I think direct user exposure to a filesystem with a POSIX-like interface is a paradigm mostly on the way out anyway, meaning it's increasingly feasible to design systems to not exceed a safe utilization threshold.
`tar` and `zfs send | zfs recv`.