From that total of 16TB it sounds like you would be using a 4TB drive as a cache drive if you used all 6 motherboard ports. I'd say don't do that. 4TB is about as overkill as you can get for a cache disk. What I would say is use that 6th 4TB for a 5th array disk and then get a cheap $15-20 dollar 2 port SATA PCIe card and tuck away a laptop drive or a SSD somewhere in the case. I, for instance, use a 320GB laptop drive as my cache drive.

 

Agree.  And the case holds 7 drives with no problem.  The drive cage on the bottom can hold either 2 3.5" drives or 3 2.5" drives.    So with the parity drive installed on that cage (as I suggested), there's room for one more drive of either size.    An inexpensive PCIe card with 2 SATA ports (< $25) would let you add a cache drive without using any of the motherboard ports.  But it would also let you have 20TB of protected storage 

I haven't really looked to much into cache drives.  I understand the general idea of them, but haven't really seen what is good/bad/ideal for one.  I realize that it doesn't need to be massive like the other ones.  When I said 16TB, I meant for all 6 disks, 4 would be data, 1 parity, and 1 cache.  Of those 6, parity would have to be a 4TB, and ideally I would have all 4 data drives at 4TB for a total of 16.  That leaves 1 more drive (whatever it should be, I may grab one of my spare drives for it) for the cache.  Of course I can use a PCIe card for a 7th drive, I feel a little worried about heat issues.  It's something I will have to monitor and decide when I get to that point in my build.

 

I suppose on that note, what is "ideal" for a cache drive? Should I be looking for High RPM, or even something in the SSD category or will just something with a decent amount of space (I have a Laptop HDD with 320gb that I'm not using) suffice?

 

If you're happy with a 16TB array max, then indeed you could simply use the last motherboard port for your cache drive.    I wouldn't be concerned with heat issues -- that case is VERY well ventilated.    My drives almost never get over 30 except during parity checks ... and then are in the low 30's.  [WD Reds ... the Seagate 4TBs likely have a different temperature profile;  but they'll still be fine]

 

Personally, I don't use a cache drive -- I want all my data fault tolerant as soon as I write it to UnRAID.    However, even if you don't want to cache your array writes, you may want a cache drive to store data for some of the add-ons you plan to use (I don't use any of those).

 

My drives almost never get over 30 except during parity checks ... and then are in the low 30's.  [WD Reds ... the Seagate 4TBs likely have a different temperature profile;  but they'll still be fine]

 

As long as the Seagate 4TB is a DM and not a DX, and everyone seems to be getting DMs these days, then it will actually probably run cooler than a Reds do. On average mine runs 5 degrees cooler than my 3TB WD Greens do, which the Reds are closely related too. So in my opinion he actually has less to worry about in terms of heat with the 4TB than he would with his original choice.

 

As to the ideal cache drive the OP asked about. I'm using a 7200RPM WD Scorpio Black laptop drive. Laptop drives, as a rule of thumb, run cooler than desktop drives do. Even in my old case where it was in a place that had literally zero airflow going over it I never saw it hit even 30 degrees. Laptop drives can also handle sustained high temperatures better than desktop drives. Glass platters, vs aluminum platters in desktop drives, being one of the reasons. They're designed to be in confined, enclosed areas with no air flow and survive.

My drives almost never get over 30 except during parity checks ... and then are in the low 30's.  [WD Reds ... the Seagate 4TBs likely have a different temperature profile;  but they'll still be fine]

 

As long as the Seagate 4TB is a DM and not a DX, and everyone seems to be getting DMs these days, then it will actually probably run cooler than a Reds do. On average mine runs 5 degrees cooler than my 3TB WD Greens do, which the Reds are closely related too. So in my opinion he actually has less to worry about in terms of heat with the 4TB than he would with his original choice.

 

 

The WD Greens are not "closely related" to the Reds.  The Reds have a significantly different power profile -- using, on average, 27% less power than the Greens -- and much lower power than the Seagate 4TB units (41% less during I/O, and 18% less when idle).    Nevertheless, as I noted, the temps should be fine with the Seagate 4TB units.    But the Red series is a FAR better series than the Greens in several respects -- using much less 12v power (250ma vs. 1.78a), and dissipating far less heat due to lower overall power consumption.

 

FYI, the average idle power consumption for the Seagate 4TB is 5W,  for a 3TB WD Green it's 5.5W, and for a 3TB WD Red it's 4.1W.  As I'm sure you know, power consumption translates directly into heat generation      Seagate doesn't publish its "spun down" (sleep) consumption;  but for the WD units it's 0.8W for the Green, 0.6W for the Red.

 

The R/W numbers are appreciably higher for the Seagate units -- but these are far less of a concern, since the % of time even a spinning drive is actively reading/writing is fairly small, except during parity checks.  The Seagate 4TB units consume 7.5W, the WD Greens 6.0W, the WD Reds 4.4W.

 

Bottom line:  While the drive temps are clearly influenced by other factors (airflow, conductivity of the mounting surface, ambient temperature, etc.), given the same conditions, the temperatures will be better for a drive that consumes less power.    So in virtually ever case, for these 3 drives, the WD Red will run coolest;  the Seagate 4TB next; followed by the WD Greens.  Clearly this is "on average" -- even within a series you'll find drives that run at slightly different temps due to manufacturing tolerances, etc.

 

The WD Greens are not "closely related" to the Reds.  The Reds have a significantly different power profile -- using, on average, 27% less power than the Greens

 

Source?

 

Granted I only ever read the initial reviews when they were just released but all the initial reviews of the Reds showed power consumption and other performance characteristics nearly identical to the WD Greens.

 

Regarding power consumption I can tell you my 4TB Seagates run 5 degrees cooler than my 3TB WD Greens. This is, literally, with a 4TB Seagate directly next to a 3TB WD Green in Norco 4224 case drive bays. So either the 4TBs use less power or power consumption is not related to heat. You could be looking at the power consumption of DX drives which are 7200RPM. DM drives are 5400RPM.

 

 

EDIT: Here is the review I was referring to. http://www.storagereview.com/western_digital_red_nas_hard_drive_review_wd30efrx

The WD Greens are not "closely related" to the Reds.  The Reds have a significantly different power profile -- using, on average, 27% less power than the Greens

 

Source?

 

The specification sheets from WD and Seagate 

 

Regarding power consumption I can tell you my 4TB Seagates run 5 degrees cooler than my 3TB WD Greens. This is, literally, with a 4TB Seagate directly next to a 3TB WD Green in Norco 4224 case drive bays. So either the 4TBs use less power or power consumption is not related to heat.

 

The 4TB's DO use less power than the 3TB Greens -- 5W for the Seagate, 5.5W for the WD Green (as I noted above).    These are per the manufacturer's specifications (as I just noted).

 

You could be looking at the power consumption of DX drives which are 7200RPM. DM drives are 5400RPM.

 

FWIW, I just confirmed that the specifications I was looking at on Seagate's site are for the DM drives