> I don't think "accurate equations" are applicable in this case.
> You can have estimates like "no more/no less than X" based on,
> basically, level of redundancy and its overhead. ZFS metadata
> overhead can also be smaller or bigger, depending on your data's
> typical block size (fixed for zvols at creation time, variable
> for files); i.e. if your data is expected to be in very small
> pieces (comparable to single sector size), you'd have big
> overhead due to required redundancy and metadata. For data
> in large chunks overheads would be smaller.
> This gives you something like "available space won't be smaller
> than M disks from my M+N redundant raidzN arrays minus O percent
> for metadata."
> You can also constrain these estimates' range by other
> assumptions like expected dedup or compression ratios,
> and hope that your end-users would be able to stuff even
> more of their addressable data into the pool (because it
> would be sparse, compressable, and/or not unique), but
> in the end that's unpredictable from the start.

Totally agree. 
We have the similar experimental practice.
It varies case by case.


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