The number of channels in your RAID card and how fast they run sets the upper limit for how fast you can access your storage.
How many disks per channel you need to provide that storage depends on what kind of I/O this server will be providing. If you're going to be storing things like workstation disk-images, you'll hit the performance ceiling a lot faster than if you were storing massive amounts of itty bitty files accessed randomly.
For significantly random I/O, disk rotational speed has an impact on your disk:channel ratio. You'll need to provide more 7.2K RPM disks than 10K RPM disks yield to the same performance.
As for SATA-600 (or 6gb SAS), if this RAID server will be connected to the network with 1Gb Ethernet, the different doesn't matter much at all. The network will saturate before the storage channels will. So take into account how your storage consumers will access this storage. It may be that a single channel with 72 drives is all you need. Or, if you have 10GbE, four channels with 24 disks each may be needed.
When it comes to buying your disks, take a look at the warranty period. Drives marketed for enterprise use are rated for 24/7/365 operation, where desktop class drives aren't. This matters most in the cheap 7.2K RPM market segment; drives at 10K or 15K RPM are almost always "enterprise" drives.
When building your RAID sets, keep RAID5 rebuild times in mind. 6TB takes a long time to rebuild, days sometimes, and performance will be degraded during the rebuild. It's better to have more, smaller R5 arrays in a stripe-set than fewer, larger R5 arrays in a stripe set.
SAS vs SATA
Doesn't matter, in my opinion anyway. SAS has a few points going for it that make it better to work with for large storage systems (>48 drives, for instance). A 7.2K RPM SAS drive will perform nearly identically to a 7.2K RPM SATA drive. The market forces an artificial segmentation, where anything 10K or 15K RPM is almost always SAS and 7.2K is mostly SATA. This is where most "SAS vs SATA" arguments are actually focusing on, drive rotational speeds.
Is it possible to mix SATA and SAS drives on the same controller?
Yes.
These are intended to be used these in server 2012 R2 software raid?
Is that a question? ;-)
Obviously not mixed in the same array but maybe on the same controller, depending
on how many disk controllers the server comes with.
SATA drives on a SAS controller / SAS hostadapter / SAS based RAID card should work just fine. You probably want to avoid port multipliers though because if a SATA drives dies then that might lock up the multiplier. If you just use direct connections then mixing them should be fine.
My original intention was a layout like this: 2 x SATA 250GB mirrored -
for Hyper-V partition 4 x SAS software Raid 1+0 - for virtual machines and
all data 1 x SATA - for backup purposes
Backups are good. But I recommend also storing backups off-line or at least elsewhere. This to prevent loss of both originals and backups in the case of fire, flooding, lightning strike, theft, ...
Problem is now it comes to ordering the hardware it seems there is some question as to whether it can support SATA and SAS at the same time.
It can. I am writing the answer from a desktop with a 3ware 9750 RAID card with both SAS and SATA drives attached to the same card.
FYI: Link to a similar question over on [SU], which was answered by a regular on [SF]. The same answer is true for software RAID. (In fact, if you are going to use SW based RAID and have a HW RAID card, then reflash the RAID card to a target/initiator mode).
Best Answer
If you want JBOD, why do you search for the raid card? You may get much cheaper HBA i.e. LSI SAS3081E-R.