I'm considering FreeNAS; and want to confirm that my understanding of what I can/can't do with storage in it is correct beforehand.
Initial build out: 1 storage pool with 2 x 6TB drives mirroring each other; for a total effective capacity of 6TB (ignoring rounding and overheads).
First expansion (2.5 – 3 years out): Add 2 x 12TB drives to the server. Configure them as a second mirrored pair, and add them to the existing storage pool; increasing my available storage to 18TB.
Second expansion phase 1 (5.5 – 7.5 years out): Add 2 x 24TB drives to the server. Configure them as a mirrored pair, and add them to the existing storage pool; increasing my available storage to 42TB.
Second expansion phase 2 (immediately after phase 1): Have all data re-balanced off of the 6TB drives, remove them from the pool, and then physically from the server. Remaining available storage 36TB.
My thoughts here are:
A doubling of needed storage capacity every not quite 3 years is a continuation of my experiences with WHS servers from 2008 to present.
After adding the 24 TB drives, the 6 TB ones will only provide a negligible fraction of total storage (1/7th) and are getting old enough that reliability will become a more significant concern if I keep them in (wrong side of bathtub curve). If they survived, at my rate of growth would only extend the time before I'd need to buy 48TB drives by a bit more than half a year; so they wouldn't really get me much time.
Limiting myself to 4 drives lets me use a compact mini-ITX form factor for my nas. Going above that means a larger and more expensive setup. (2 drives sitting on the top of an open case with wires snaking out is acceptable for a day or two of transition time; but not longer term.)
I'm also assuming business as usual for the availability of larger drives like they have been for my previous about 3 year upgrades: 1.5-3tb (2012) and 3-6TB (now/near future). And that whatever new drives do become available will be reliable enough to be usable (ie the raid apocalypse never happens).