Does FreeBSD's ZFS reserve 1/64 of the capacity for COW?

[Mark Felder]

On Fri, Dec 6, 2013, at 6:46, Trond Endrestøl wrote:
> Hi,
> 
> Today I came across "Becoming a ZFS Ninja Part 1" 
> http://www.youtube.com/watch?v=tPsV_8k-aVU featuring Ben Rockwood who 
> stated that ZFS reserves 1/64 of the capacity for COW safe-guarding. 
> Fast forward to 00:49:44. Thus, one should take this fact into account 
> when planning the storage capacity of your (larger) zpools.
> 
> Does FreeBSD's ZFS also reserve 1/64 of the capacity for COW?
> 
> If it's true, then I guess this should be noted in zpool(8) and in the 
> Handbook.
> 

Well, the rule-of-thumb / urban legend tends to be "don't let your zpool
go over ~80% or you'll have severe performance issues" so I'm not sure
that statement is accurate or COW wouldn't be so painful when your pool
is over ~80%. I could be wrong though; I often am :-)