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EVGA Z370 FTW Question Regarding M.2 Key-M Slots

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hpwitten
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2018/06/10 06:28:54 (permalink)
Good morning, folks.
 
I'm thinking about doing a new build in the near future, and I'd like the EVGA Z370 to be part of it.
I'm also looking at possibly using an m.2 SSD in the build, in an attempt to cut down on cable mess.
 
However, the manual isn't very clear on one particular point.
 
It says "M.2 is an SSD standard, which uses up to four PCIe lanes and utilizes Gen3 speeds. Most popularly paired with NVMe SSDs, this standard offers substantially faster transfer speeds and seek time than SATA interface standards."
 
Which is fine, but in the same bullet point, it says "Each Key-M shares lanes with SATA ports 0/1 or 4/5, respectively," and that seems like a bit of a contradiction. It either goes across the PCI-e lanes in the northbridge, or across the SATA lanes in the southbridge. This is confused further by the Key-E text, which specifically notes that it will disable the third PCI-e slot if it's occupied.
 
So my question is, for the M.2 Key-M slots, will it suck up PCI-e lanes or not? And, also, since each one disables two SATA ports, does that mean it bonds the two for double the bandwidth?
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    Cordorb
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 07:29:36 (permalink)
    That about what you get ( making serial faster by going parallel )
    there is a better list posted of which ports are used/disable in BIOS for which board types by EVGA  
    The manual's will also show that as well as the board art work.
     
    At least this make it easy to go back and forth - if you add or remove a M.2
     
     
    now if you use a non- NVMe SSD M.2 ie a SATA only M.2 I don't know if the BIOS still takes all 4 lanes ?
    I only tried that on a EVGA Z270
     
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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 09:04:49 (permalink)
    NVMes go through the Z370 PCH (e.g. "Southbridge"), just like all other IO devices do -- they do not go through Northbridge.  And so NVMe's use/share the same 30 available PCH HSIO lanes used for USB 3.0/3.1, PCI-e, SATA, NVMe, etc. devices.  NVMe is just another PCI-e device (that can use either 2 or 4 lanes); SATA drives use one PCI-e lane (but are further limited/constrained by the SATA bus speed itself, and not the "upstream" PCI-e lane).  And so NVMe's and SATA SSDs must share the same 24 available PCI-e lanes reserved for IO.  (Note:  The PCI-e lanes for Northbridge are separate from these 24 "IO" PCI-e lanes and are reserved for GPU(x16 lanes) & RAM (x4 lanes, I think) only).
     
    This picture may help show where HSIO lanes are being shared between/among IO devices, such as NVMe's (shown as "Intel PCI-E Storage Device) and SATA ports/drives: https://www.tweaktown.com/guides/8482/intel-z370-motherboard-buyers-guide/index.html 
     
    You can see in Figure 3-1 where an "Intel PCI-E Storage Device" (e.g. NVMe SSD or Optane) shares lanes with SATA ports.  A SATA port will use only 1x PCI-e lane, whereas NVMe SSDs can either use 2x or 4x PCI-e lanes, depending on the NVMe drive.  Of course, if you plug in an M.2 SATA SSD, it is really no different than a normal SATA SSD and uses the same 1x lane (only difference is M.2. form factor).
     
    So if you plug in an NVMe that uses PCI-e X4 (like the Samsung 960 Evo NVMe, PCI-e X4), then it will use all 4x PCI-e lanes, and thus the 2x (both) SATA ports that were sharing those lanes must be disabled.  BTW, my mobo lets me configure one of my NVMe slots to either "X2 Mode" or "X4 Mode" ... to correspond with X2 NVMes and X4 NVMes.  So if I were to plug in a X2 NVMe (like the ADATA XPG SX6000 M.2 NMVe, PCI-e X2), then the shared SATA ports are still enabled (that is, of the 4x PCI-e lanes, 2x will be used for the X2 NVMe, and 2x will be used for SATA5/6 ports).  This options gives me a little flexibility on that M.2 port.  My Samsung 960 Evo NVMe was plugged into it but was defaulted in bios to running at X2 mode ... so I changed it to X4 mode and drive performance went up significantly--and disabled 2x SATA ports (5/6).
     
    So the above explains why if you fill one or both M.2 Socket 3 Key-M slots, that you will correspondingly disable SATA ports.
     
    Likewise, the M.2 Socket 1 Key-E only uses 1x PCIe-lane meant for lower speed M.2. IO devices, that it shares with the PCI-e X1 slot #PE3 (the middle one, if I'm not mistaken).  So if you plug in a low-speed M.2 IO device here, it will disable PE3.
     
    Lastly, mobo makers can tailor their mobos to distribute the HSIO lanes a little differently.  I can't find the HSIO breakdown/figure for the EVGA Z370 FTW.
     
    Hope this helps.
     
    post edited by thunder-93 - 2018/06/10 09:16:36
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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 09:11:39 (permalink)
    hpwitten
    So my question is, for the M.2 Key-M slots, will it suck up PCI-e lanes or not? And, also, since each one disables two SATA ports, does that mean it bonds the two for double the bandwidth?



    Kind of "yes" in that if you install an M.2 NVMe SSD, the mobo (Z370 chipset) will allocate/dedicate X4 PCI-e lanes to it (thus disabling 2x SATA ports).  So X4 PCI-e lane bandwidth will be available to the SSD.  Whether the M.2 NVMe SSD itself can utilize that bandwidth depends on the specific M.2 NVMe SSD itself, whether it is X4 (and can use all 4 lanes) or X2 (and will only use 2 lanes ... performance a little faster than SATA SSD but slower than X4 NVMe).  This is why X2 NVMes cost less, are a little faster than SATA SSD, and get rid of cables (which is why these budget NVMes are quite popular).  This is also why X4 NVMes cost more (requires higher-end on-board controller to take advantage of the X4 lanes) and are much faster than the other SSDs.  

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    hpwitten
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 13:50:50 (permalink)
    thunder-93
     
    Kind of "yes" in that if you install an M.2 NVMe SSD, the mobo (Z370 chipset) will allocate/dedicate X4 PCI-e lanes to it (thus disabling 2x SATA ports).    




    Okay, so that's in the case of an NVMe SSD.
     
    But if we're considering , where it says it's an m.2 SATA SSD, why would it occupy PCI-e lanes if it's SATA? That's the part where your first reply lost me a little; my understanding is that the 16 PCI-e lanes addressable by the CPU refers specifically to the northbridge's lanes, whereas the southbridge, where SATA lives, doesn't talk over PCI-e at all.
     
    Why are we talking about SATA lanes and PCI-e lanes like they're the same thing? Did something change in Intel's implementation recently?
     
    EDIT:
     
    I suppose ultimately it doesn't matter all THAT much, as even if something does occupy the PCI-e lanes, it'll only knock a GPU back to x8, and real-world data shows there's no difference, under average workloads, between x8 and x16...but now I'm confused and curious as to the answer.
     
    And also, I suppose, whether m.2 is superior to standard formfactor SATA. That question, I haven't found data on.
    post edited by hpwitten - 2018/06/10 13:58:44
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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 16:43:57 (permalink)
    Point of clarification ... there is no Northbridge/Southbridge under the Coffee Lake architecture ... Northbridge/Southbridge is older architecture that was replaced in 2011 by Sandy Bridge; which was replaced by Ivy Bridge; then Haswell; then Broadwell; then Sky Lake; then Kaby Lake; and now Coffee Lake microarchitecture.
     
    COFEELAKE:  Today, the Intel 8th gen CPUs (like the i7-8700K) have 16x dedicated PCI-e lanes for the CPU to communicate with GPUs (using 16X lanes) & RAM (using 4X lanes).  In *addition*; the Intel Z370 host chip itself has its own 24x dedicated PCI-e lanes set aside for IO devices only.  (H370 has 20 PCI-e lanes, B360 has 12 PCI-e lanes; H310 has 6 PCI-e lanes).  So you have a total of up to 40 PCI-e lanes using the Z370 chipset.
     
    hpwitten
    I suppose ultimately it doesn't matter all THAT much, as even if something does occupy the PCI-e lanes, it'll only knock a GPU back to x8, and real-world data shows there's no difference, under average workloads, between x8 and x16...but now I'm confused and curious as to the answer.

     
    GPU:  This is not correct.  Storage devices do NOT use any of the PCI-e lanes that are dedicated to GPUs.  GPUs have their own separate/distinct/reserved PCI-e lanes--16X of them direct with the CPU.  Storage devices (and other IO devices) communicate with the Z370 host chip which has its own dedicated 24X PCI-e lanes.  So plugging in any drive (SATA, NVMe, SSD, HDD) would utilize the 24X PCI-e lanes from the Z370 host chip and NOT the 16X PCI-e lanes reserved for GPU.
     
    hpwitten
    And also, I suppose, whether m.2 is superior to standard form factor SATA. That question, I haven't found data on.

     
    FORM FACTOR:  M.2 form factor (e.g. chip w/no cables) is neither superior nor inferior to SATA form factor (2.5" with cables) -- it's just different.  It should not be confused with the protocols that run on it:  for SATA devices the AHCI protocol which was built for mechanical HDDs and thus is slow, and for NVMe drives the NVMe protocol which takes advantage of PCI-e lanes and speeds--more lanes equals more speed potential.
     
    hpwitten
    Why are we talking about SATA lanes and PCI-e lanes like they're the same thing? Did something change in Intel's implementation recently?

     
    SATA and PCI-e are not the same thing.  HSIO and PCI-e are also not the same thing.  I think here-in lies the confusion.  I will try to explain more clearly in my next post.

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    hpwitten
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/10 18:40:12 (permalink)
    Ohhhhhhhhhhhhhh.
     
    So the source of the confusion is I'm an idiot and thought there was still two distinct bridges. That's what had me confused, because I heard "PCI Express lanes" and associated such lanes only with the GPUs.
     
    Whereas in actuality, what's happening is more like this: The two silver slots  sit on those 16 GPU lanes, and literally everything else -- the four black slots, SATA drives, the NIC, USB ports -- they all communicate with the chipset over the remaining 24 lanes. So I could throw an NVMe drive in the topmost M.2 E-key slot and not impact the GPU's bandwidth at all, and ditto with the lower M.2 M-key slots -- the only difference is they leverage AHCI?
     
    Or...does the physical port not care in this case? Could I buy, say, the Samsung 970 EVO, which brands itself as being NVMe, and connect it there? The fact that the slots specifically talk about disabling the SATA ports leads me to an initial assumption that I would not be able to do that.
     
    thunder-93
    COFEELAKE:  Today, the Intel 8th gen CPUs (like the i7-8700K) have 16x dedicated PCI-e lanes for the CPU to communicate with GPUs (using 16X lanes) & RAM (using 4X lanes).   
     



    Does this mean that the RAM will take four lanes from the 16 provisioned for the GPU, leaving 12? Or am I misunderstanding this, as well?
     
    I apologize for the ignorant questions -- I haven't needed to consider the low-level implementation of chipset architecture in years.
     
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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/12 16:27:37 (permalink)
    hpwitten
    Whereas in actuality, what's happening is more like this: The two silver slots  sit on those 16 GPU lanes, and literally everything else -- the four black slots, SATA drives, the NIC, USB ports -- they all communicate with the chipset over the remaining 24 lanes. So I could throw an NVMe drive in the topmost M.2 E-key slot and not impact the GPU's bandwidth at all, and ditto with the lower M.2 M-key slots -- the only difference is they leverage AHCI?

     
    Close.  So think of the Z370 host chip having 30 HSIO lanes (High Speed Input/Output lanes).  And "on top" of these lanes can run other interfaces--for example USB, PCI-e, SATA, network, etc.  So if you look at figure 3-1 again, you can see HSIO lanes #1-6 (labeled in #s in orange on top) are dedicated to USB only--and only USB runs on these particular HSIO lanes.  If you look at HSIO lanes 7-30, you can see that all these lanes support PCI-e (24 of them for Z370), with some of them being only PCI-e, while others can share with other interfaces (USB, SATA, ethernet, for example). 
     
    -- For example, HSIO lanes 27-30 only support PCI-e (depicted as PCI-e lanes #21-24) for PCI-e devices only. 
     
    -- Another example would be HSIO lanes #23-26 which supports both PCI-e (PCI-e lanes #17-20) and/or SATA interfaces (SATA #4 and #5).  So to use these HSIO channels, you could plug in 2x SATA devices into SATA ports #4 & 5; or you could also plug in an M.2 SATA SSD which would also use one of these SATA interfaces.  You could also plug in an M.2 PCI-e **X2** NVMe (which would use PCI-e channels #19 & 20, "on top of" HSIO lanes #24 & 25)) _at the same time_ as the 2x other SATA devices (using SATA5 and SATA6, "on top of" HSIO lanes #23 & 24).  OR you could plug in an M.2 PCI-e **X4** NVMe, which would take up HSIO lanes 23-26 and thus PCI-e lanes 17-20 ... which would disable SATA ports 4 & 5 at the same time.
     
     
    hpwitten
    Does this mean that the RAM will take four lanes from the 16 provisioned for the GPU, leaving 12? Or am I misunderstanding this, as well?

     
    Not quite.  The GPU(s) has 16x PCI-e lanes set aside for graphics only--these are not shared with anything else.  If you have one CPU, it gets all 16 lanes.  If you have 2x GPUs and SLI, for example, then each would get 8 lanes.
     
    And RAM has it's own separate memory interface and channels to communicate between the CPU and RAM--I don't know the protocol.  And the memory controller on the CPU controls communications with RAM, to include "overclocking" the RAM with XMP, for example.  This is why the ability to overclock RAM (and how far) is dependent upon the CPU, and not just the motherboard.
     
    Lastly, chipsets determine the # of HSIO lanes available (Z370 with 30 HSIO lanes, including 24 PCI-e lanes), and motherboard makers make decisions on how to allocate those lanes for IO.  They can choose how many M.2 slots to support (1, 2, or 3), whether each slot can support M.2 SATA only, M.2 PCIe/NVMe only, or both (for example, my mobo has 2x M.2 slots, one of which supports M.2 SATA or M.2. PCI-e/NVMe while the other one only supports PCIe/NVMe (can't plug an M.2 SATA into it).  And this is how different Z370 mobos, for example, support different #s of M.2 devices, or have different # of SATA devices, or different # of PCI-e slots (of different sizes, X16, X8, X4, or X1).  EDIT:  Also, this is where H370, B360, and H310 host chipsets support fewer HSIO lanes, and thus fewer PCI-e lanes, and thus fewer devices.
     
    A very long answer to a short question.  Does this make more sense?
    post edited by thunder-93 - 2018/06/12 16:41:59

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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/12 16:38:47 (permalink)
    hpwitten
    Or...does the physical port not care in this case? Could I buy, say, the Samsung 970 EVO, which brands itself as being NVMe, and connect it there? The fact that the slots specifically talk about disabling the SATA ports leads me to an initial assumption that I would not be able to do that.

     
    Physical ports are mapped to specific HSIO lanes, which support specific interfaces "on top".  So you won't be able to plug in the Samsung 970 EVO NVMe into an M.2-Key E slot, but you can plug into an M.2-Key E slot an M.2 wifi card (this slot is for wifi M.2 only).  And you can plug in the Samsung 970 EVO NVMe into either M.2-Key M slot, since they both happen to support either M.2 PCI-e or M.2 SATA.  (see page 34 of the manual for reference).

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    hpwitten
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/13 05:05:01 (permalink)
    I think I had a major breakthrough, actually, in my own naive understanding - thanks to your patience and that Tweaktown link.
     
    My new understanding is more like this:
     
    The 8700k CPU itself features the sixteen interconnection lanes which power the GPU, and aren't exposed to anything else in the system.
     
    Additionally, the Z370 *chipset* features an additional thirty, which connect to all other peripherals and I/O. These lanes are completely separate from the sixteen noted in Intel's ARK database, and also separate from whatever transport mechanism links to RAM.
     
    The chipset's thirty HSIO lanes are broken down as follows:
     
    The first six must be used for USB3.
     
    The following 24 are the "PCI Express" lanes that aren't the sixteen directly linking the CPU to the GPU (I think those sixteen used to be called Quick-Path Interconnect?)
     
    These 24 lanes, though they rest on top of PCI-E, can have other services put on top of them -- I'm immediately put in mind of the OSI networking model, where PCI-E might be thought of as the Transport layer, with Session/Presentation/Application still "on top," representing multiplexing into further USB lanes, SATA lanes, Ethernet, etc.
     
    Some of the Z370 FTW's PCI-E lanes are electrically switched between SATA ports 0/1 and 4/5 and the two m.2 Key-M slots in a mutually exclusive fashion.
     
    (The manual says each slot is x4, but each SATA port occupies one HSIO lane, so I'm not entirely sure what's going on there.)
     
    But since the SATA / AHCI protocols "ride on top" of the underlying PCI-e lane, I can stuff an NVMe SSD there and take advantage of the full bandwidth thus exposed.
     
    I think I've gotten that all right?
     
    It really isn't set out that clearly, though, which is kinda BS. But thanks a lot for your explanations!
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    thunder-93
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    Re: EVGA Z370 FTW Question Regarding M.2 Key-M Slots 2018/06/13 18:48:29 (permalink)
    Yes, I think you've got it.
     
    And yes, I have the same question regarding each SATA port using 1x HSIO lane.
     
    Also, is that different mobo makers can change how they choose how to divide up the 30 HSIO lanes for a Z370 chipset across different mobos.  Thus they can have some different features (mainly IO devices onboard or supported).  For example, my Asus Z370-E mobo for one of the M.2 ports allows me to tune it to PCI-e X2 or X4 ... so in X2 mode it does not disable the shared SATA ports (in X4, it does disable them since the X4 NVMe will use 4 lanes by itself).  For mobos with on-board wifi, it will use HSIO lanes for the wifi ... and those without on-board wifi will have additional HSIO lanes for other IO devices.
     
    Lastly, is the same framework applies to H370, B360, and H310 mobos ... they just have fewer HSIO lanes (and thus the cost savings) along with fewer PCI-e lanes.
     
    I agree it is confusing.  Probably because I'm not a computer engineer.  

    Great chatting with you!  I learned a lot with our discussion!

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