I found this post while trying to understand why my brand new 20TB drives, freshly formatted in Unraid 7 with XFS defaults, were starting off with 383GB allocated - much higher than the documented expectation of 0.7%.
I found the analysis here very useful, and tested formatting with reflinks disabled, and am now at 263GB allocated (confirming reflinks consumes 6GB per TB). I'm still not sure if I need reflinks or not - from the discussion here it seems I likely don't for my use case.
But what isn't answered here is why crc alone is roughly double the documented allocated size (for 20TB it seems I should be around 140GB). I wonder if perhaps there was an enhancement with XFS crc in Unraid 7, as I don't see many older posts showing anything close to this.
Has anyone else been able to figure this out?