4x3TB RAID-Z1 Producing 7.8TB, not 9TB

[EricDKaufman]

I just finished formatting a 4x3TB RAID-Z1 system with FreeNAS. I choose Z1 because it was the closest to 5 and clearly superior is write hole error suppression. RAID 5 should generate 9TB from 12TB, so why am I only getting 7.8TB from Z1? Are my 'lowest bidder' 3TB drives secretly 2.7TB (lol, facetious)???

 

[ben]

Drive manufacturers use base-10 gigabytes and terabytes. Operating systems use base-2 Gibibytes and Tibibytes and call them gigabytes and terabytes. Drive manufacturers are being deceptive-ish, but at least they're following the convention. It's not false advertising, just poor communication.

 

[StephenFry]

Yeah, for all intents and purposes, 3TB drives are in fact, 2.7TB.

Don't worry though, this is normal, as ben explains. Look at your pool with a system that reports the actual usable bytes. It will be over 8.900.000.000.000.