Ya I was planning on putting them all into one vdev unless someone tells me that's a bad idea..
You have to decide whether it is a bad idea - it is your data so only you can tell how safe the data should be.
From what I know/read, it is possible to have more than 12 HDDs per vdev - many more. However, it is adviced to not do that (every RAID gets unsafer the higher the hdd-count is).
Having 12 HDDs, I see two scenarios:
a) 1 vdev consisting of RAIDZ3 12x 4 TB = 9*4TB = 36TB * 0.8 = 28.8 TB usable
b) 2 vdev consisting of 2x RAIDZ2 6 x 4 TB = 2x (4x 4TB) = 32 TB * 0.8 = 25.6 TB usable
Thoughts on both configurations:
- the throughput depends on the number of HDDs - so there should not be a significant difference
- the IOPS depend of the number of vdevs in the pool - so (b) has higher IOPS (needed for simultaneously accessing many files (typically databases))
- (a) is safe against three drive failures
- (b) is safe against two drive failures in each vdev (so there can fail 2x2 drives if you are lucky or only 2 if you are unlucky and they are in the same vdev)
Reading your questions I'd recommend to read cyberjock's guide again in all detail and
take your time. I understood the important zfs-information from reading that ppt carefully.
As for HDD, guessing you are going for storage and not IOPS, I recommend against 7200 rpm drives (hotter, power consumption) and for 5XXX rpm drives.
This is because your FreeNAS is likely connected via a GB-LAN to the outside and thus offers at best around 110 MB/s.
Any RAID-Z with three drives will saturate that network link.
As for HDD brands, I can tell you one thing: All HDDs fail sooner or later.
I'd currently go with Seagate's Desktop HDD.15 ST4000DM000 due to the good price tag and latest reliablilty report by
backblaze.
Another common choice is the WD RED 4 TB.
Good luck!