Does the 80% capacity rule of thumb for ZFS pools apply regardless of the size of the pool? If I have a pool of 10TB, that means I have to keep 2TB free. Fair enough, the loss isn't too great. But in a 50TB pool, that means I need to keep 10TB free. That's a lot of free space left over...


Yes. The 80% rule applies... (with some exceptions)

Think about it the same way you'd approach monitoring of a volume. 90% full is 90% regardless of the actual capacity and would still yield an alert.

This is no different. You don't want to plan or expect to run at that high of a capacity.

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    I don't know about you, but I don't care about a volume being X% full, so much as I care about, "is this going to fill up and cause me grief any time soon?" If I've got a 100TB volume, with a usage growth rate of, say, 10GB/day, and a week's lead time on provisioning new capacity, I'm not going to give a flying fig when it's got 10TB (or 20TB) free. I'll start to care when it's down to 1-2TB free.
    – womble
    Nov 6 '15 at 2:18
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    Except copy-on-write filesystem performance tends to degrade heavily above 85% usage. If snapshots are involved, it's VERY easy for an inadvertent action to exhaust filesystem space. This is why people strongly suggest remaining below 80% on ZFS filesystems.
    – ewwhite
    Nov 6 '15 at 2:21
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    Sure, but that's unrelated to an analogy of monitoring volume capacity.
    – womble
    Nov 6 '15 at 3:07
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    No it isn't unrelated - it's directly saying performance degrades above x% regardless of how long it would take to fill the remaining space, so there's a specific reason to care. You can still say "I don't care about that" but that implies you don't give a flying fig about storage performance... Nov 6 '15 at 3:17

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