Well, the GUI warning about capacity of the pool is based on #1, the raw drives.
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https://github.com/freenas/freenas/blob/master/gui/system/alertmods/zpool_capacity.py
Cool, thanks for looking that up :)
ZFS won't ever shred data, but it does have a failure mode where if you actually manage to fill a pool to 100%, you might not be able to commit the writes that are needed in order to update metadata to remove files to clear space. So you really do want to avoid hitting 100%.
Good to hear about the not shredding data ;) As for 100%, I will take this opportunity to remind people about space reservation.
95% of pool capacity, period. See "zpool list."
Awesome, thanks :)