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Old January 7th 19, 02:58 AM posted to alt.comp.periphs.mainboard.asus
Bill Anderson
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Posts: 249
Default All I wanted was just to use that M.2 socket

On 1/6/2019 3:11 PM, Paul wrote:
Bill Anderson wrote:
I have an Asus Prime X299-A board with an i7-7820X 8-core processor
loaded with six various sized SATA drives plus a 500GB Samsung 960
PCIe 3.0 x4, NVMe 1.2 EVO drive that serves as my main boot drive.

There are two M.2 sockets on the board: the one I use with my M.2
Samsung NVMe SSD is pointed vertically and is easily accessed any time
I want to remove the drive, and the other sits pointed horizontally
under a 3-screw metal plate that can't be removed without first
removing the graphics card -- not easy to access at all.

But that hidden M.2 slot is sitting there empty and I'd really like to
use it for storage of stuff I'd like to access at high speed. (I don't
want to use it as a boot drive.) The manual says it'll work with
either a PCIE drive or an SATA drive, so I stupidly ordered a 1T M.2
SATA drive, even though I was slightly worried I'd already maxed out
the board's SATA capabilities with an 8-core processor.Â* And yes, of
course I had -- when I installed the new M.2 drive, BIOS promptly
disabled one of my other SATA drives.Â* Drat.

So I'm going to try to send it back to Amazon, though I won't be
surprised if they refuse to take back an electronic device, even
though I've never even partitioned it.Â* We'll see.

But I accept responsibility for doing this stupid thing and I'm
resigned to paying the price if I must.Â* Still, I'm wondering...

If it turns out Amazon will do an exchange, is it possible I could run
a second PCIE drive in that M.2 socket?Â* I'm asking here because I no
longer trust my understanding of all this to make a purchase without
advice.Â* According to the manual the socket I want to use supports
"PCIe 3.0 x4 and SATA mode M key design."Â* Does that mean a PCIe M.2
SSD would not disable one of my regular SATA drives while an SATA M.2
SSD would? Even though I want 1T for storage, I'd be willing to
exchange my 1T M.2 SATA drive for a 500GB M.2 PCIe drive, as they're
basically the same price at Amazon.

Should I take that route, or just try to get my money back if I can
and forget the whole thing?Â* Thanks.


You could look for a controller board with a chip on it that
drives a SATA M.2. But this would use one of your PCIe slots,
and that might be all that desirable (your slots are
probably all used by now).

https://www.amazon.ca/StarTech-com-M.../dp/B017IM54GM

*******

https://en.wikipedia.org/wiki/M.2

Â*Â* "The M.2 specification provides up to four PCI Express lanes
Â*Â*Â* and one logical SATA 3.0 (6 Gbit/s) port, and exposes them
Â*Â*Â* through the same connector so both PCI Express and SATA storage
Â*Â*Â* devices may exist in the form of M.2 modules."

*******

There really isn't a good reason to buy a SATA version of
an M.2. That would only run at 450-500MB/sec or so. Whereas
the PCIe x4 version of M.2 does around 2GB to 2.5GB/sec in
some cases (it can't do 4GB/sec because of the limited
size buffers on the Intel end for one thing).

With modern motherboards, it pays to find the PCH datasheet
and the section on "FLEXIO". As that will tell you what
enabling one thing will disable some other thing. However,
you still need to understand which facility of the FLEXIO
is wired to which slot or connector. Having this understanding,
compensates for motherboard manuals lacking in the proper
details. It's possible Anandtech has had copies of the
FLEXIO picture in some of their reviews.

I think it "suspicious", that the M.2 with SATA capability,
backs right up to one of the right-angle SATA stacks. That's
probably a coincidence, as the switching of the function should
be in the PCH. You can't drive two connectors
off one port, and unlike expansion cards, the motherboard
is unlikely to use jumper plugs to control which
direction the port wiring goes to (left or right). Since it's
likely the PCIe wiring and the SATA wiring share some diff pairs,
the PCH is likely routing a particular port, to a
(limited) set of SATA logic block . Likely arranged
as two identical four port controllers or something.
The FLEXIO likely needs a kind of crossbar, so wire pairs
on the PCH can be routed to the correct internal logic blocks.

I used to absolutely *hate* the FLEXIO concept when I was
doing board design. Intel isn't the first to use this,
and the concept has been used elsewhere. Some stupid chips
have five functions bound to one pin, and you sit there at
your desk for hours, juggling the damn things and trying
to get the best usage out of the interfaces. The boss is
putting on the pressure to produce, while you're whiling
away the hours playing "crossword puzzle" with some stupid
chip docs.

In the Asus manual, I can see that PCIeX4_2 shares with
SATA5678, but no mention is made of the M.2 needing to
rip off a port on one of the controllers.

And you have a 28-lane CPU, with the restrictions that gives.

I'd probably start by verifying that moving the x4 boot
drive over to the other M.2 slot works without disabling
anything else. And then look into exchanging (or paying
a restocking fee) for the M.2 SATA flavored device. The
thing is, even if you used a SATA adapter card in an
attempt to get some usage from the SATA M.2 somehow, that
uses a PCIE slot, and maybe that would have adverse consequences
on yet some other sharing inside the PC.

Somehow you're going to have to figure out how the
FLEXIO works. It's possible that M.2 slot fully
terminates on the PCH, and it's the PCH crossbar
that moves the internal SATA block from using
the right-angle conventional SATA connection, over
to the M.2 slot.

Â*Â* Paul


Thanks, Paul. Apparently returning the drive to Amazon will be no
problem at all -- I'll pay just for return UPS shipping. I went ahead
and ordered a different drive -- it's a Crucial PCIe 1T and it's only
about $30 more than the SATA drive was. I figure it it doesn't work
either, it'll cost only the shipping fee again. I can handle that.
Thanks for all your help!

--
Bill Anderson

I am the Mighty Favog