Comment by gpt5
16 hours ago
ZFS will auto-degrade performance if there isn't enough headroom. In addition, if you use SSD, you also want the headroom, because otherwise you end up writing and rewriting on the same small empty space which kills SSD (unless you have enterprise SSD, which have built-in headroom).
Not sure why I'm being downvoted for being factually correct (and also, why I can't edit my above comment).
ZFS will definitely degrade write performance gradually from 90% utilization, and will hit a stronger cliff at 95%+. Same with SSD, using a consumer SSD above 90%+ utilization would rapidly degrade its lifetime. The effect will be smaller for very large pools and very large files, but the effect is stil there.
I'm repeating this so that people who set up these drives know what is actually going to happen.
> Not sure why I'm being downvoted for being factually correct
Maybe because your statement about SSDs is incorrect. Consumer SSDs have spare blocks too, and wear leveling prevents the scenario you're describing.
I appreciate the reply, but my warning consumer SSD is not incorrect. Perhaps I should have clarified that While they typically have a small overprovisioning of 7%, enterprise SSDs have close to 4 times as much, exactly to allow more writes (which is very relevant if running a ZFS raid on the drive). Keeping 10-20% headroom will prolong your SSD's life significantly, and increase performance of ZFS.
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