Adam:
I'm not familiar with the calculator you're talking about - can you provide a link?
A RG size of 24, with 3TB drives, should give you just under 60TB usable (22 data drives*3TB*.9 WAFL overhead) - but you'd want a spare, so say RG size=23, 21+2, ~56TB.
You say active/passive - but then you say "just the 2240 with 24x3tb SATA drives". Do you have one controller or two? If you have two, you'll have to allocate 3 drives (1D+2P) to the inactive controller for vol0, which leaves you a possible RG size of 21 (19+2), which gives you just over 50TB on your active system, assuming you don't split out your vol0 there. But that doesn't leave you any spares. If you leave one spare per controller, you're left with about 46TB on your active system, with a RG size of 19 (17+2). You could manage with a single spare on you active system, and assign it over to your inactive system if you need to, whice gives you back on data drive.
One thing to consider is future expansion. Once you decide on a RG size, you want to grow the aggregate by multiples of that many drives, to avoid performance hits. For example, if you go with a RG size of 23, each time you add a shelf you add another RG plus a spare, which is good. But if you go with a RG size of 19, then you add another 24 drives, you've got 5 drives either to use as spares (too many and wasted space) or you end up with an incomplete RG, which leads to poor performance.
I've done RG sizes of 22 drives, but not on SATA. It also depends on your project requirements - IOPS vs. space vs. expansion vs. redundancy...
Hope that helps...
Bill
