15 x 8TB in 4 Arrays

I am planning on using Rockstor+BTRFS on a 45 Drives XL60. To provide a growth path, assuming we will add 15 8TB drives at a time, creating an array for each of the 15 drive groups. What can I assume the amount of usable storage will be per 15 x 8TB array? Feel free to suggest alternative groupings.

@gafriedman2144 Welcome to the Rockstor community. Just a quick question, are your 8 TB drives the Seagate Archive SMR drives, or possibly other SMR drive models, if so they are not suitable for use with btrfs. Just thought I’d pop in with this one as it has come up before on the forum; just in case.
If they are please see this post in the SINGLE smr Drive Going Offline thread in this forum, and the linked post in that reference for more details.

As to your question that depends on your raid level and if you use one of the available compression options and if your data is compressible of course. There are web based btrfs calculators but the one I just found wasn’t really that friendly. I’ll have to leave this question to others for the time being I’m afraid. But that calculator should give some indication. Btrfs raid levels work a little differently to current (but traditional) raid levels though and drive groupings can be grown or have their raid levels changed on-line / in place. The general thought currently is to use either btrfs Raid1 or Raid10. Or if no redundancy is required then Raid0 to maximize the usable space.

Anyone have a link to a better btrfs space calculator?

Well, that’s a gotcha! So, I guess one of the most important considerations is this: if Rockstor+BTRFS using Raid6 loses a drive, hot swapping the spare should be a trivial exercise. Assuming that this is true, I thought using other than Enterprise class drives made sense. To save me time, are there any other lower cost 8TB drives that do not use SMR?
I found a raid efficiency calculator that indicates 86.4% efficiency with Raid6 BTW

Looks like from a price performance point of view, the 6TB Seagate ST6000VN0021 at $249 a copy vs ST8000NE0011 is the way to go. 6 cents a gig vs 4 cents a gig.