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post #1 of 7 (permalink) Old 02-28-2019, 02:05 PM - Thread Starter
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PCIe lanes for beginners

Good evening everyone,

I hope I am posting on the right forum. If not please redirect me and I will be happy to correct my mistake.

I would like to understand more about PCIe lanes. And I would like to use my current (old) build as an example to see if my 48h of googl'ing has made me any wiser (or only more confused).
My secondary goal is to determine if I can improve my SSD read/write speed in my old build by upgrading f.e. to an NVMe PCIe based SSD or that I have to buy a new Motherboard and CPU as well.

My current build:

Motherboard: Asus P8Z68-V PRO (only PCIe 2.0 is supported, NOT PCIe3.0)
Chipset on MB: Intel Z68
CPU: i7 2700 K
GPU: Radeon RX 580


<< Sources >>
------------
Block diagram: https://www.overclock.net/photopost/...ckdiagram.jpeg
PCIe speeds: https://www.embedtek.net/wp-content/...18/12/char.png
Complete MB manual: https://dlcdnets.asus.com/pub/ASUS/m...8Z68-V_PRO.pdf

Question 1: Am I correct when I say that my CPU provides up to 16 PCIe 2.0 lanes (direct link from PCIe slot to CPU)? Either 1 slot at x16 speed or 2 slots at x8 speed each.

Question 2: My motherboard furthermore provides 4 PCIe 2.0 lanes (Not direct to CPU, but rather over the Z68 chipset) ?

Question 3a: Where does the 16Gbytes/s come from on the block diagram <<see link above>> on the x16 slot ? I thought PCIe 2.0 offers 500 MB/s per lane (= 8.0 Gbytes/s for x16) ?? <<see PCIe speeds link above>>

Question 3b: Same question as question 3a. If i assume the lower 'b' here in 5 Gb/s stand for bits (does the 'Gb' stand for Gbits/s and the GB for Gbytes/s ??), this would make sense since 5 Gbits/s = 625 Mbytes/s (500 Mbytes/s if I take the 8b/10b decoding into account). And 500 Mb/s is what I find to be the speed of PCIe 2.0 per lane in the << PCIe speeds link above >>.

Question 4: Does my motherboard support NVMe protocol ? OR only AHCI protocol ? (I assume it doesn't support NVMe since it is way old)

Question 5: I read a lot about the fact that PCIe slots can be connected to the CPU using PCIe lanes OR SATA III connection. It depends on the MB manufacterer. Is this true and how can I check this ? My MB manual doesn't specify it.

I have some more questions, but I will hold them for later
I hope I put enough structure in my post for you to understand my situation.
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Last edited by 2Dex; 03-01-2019 at 08:49 AM.
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post #2 of 7 (permalink) Old 03-01-2019, 11:52 PM
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Quote:
Question 1: Am I correct when I say that my CPU provides up to 16 PCIe 2.0 lanes (direct link from PCIe slot to CPU)? Either 1 slot at x16 speed or 2 slots at x8 speed each.
Yes, per the Intel data sheet for your i7-2700k, the processor itself supports a maximum of 16 PCIE 2.0 lanes. More lanes are supported through the DMI interface and the Z68 Express Chipset.

Quote:
My secondary goal is to determine if I can improve my SSD read/write speed in my old build by upgrading f.e. to an NVMe PCIe based SSD or that I have to buy a new Motherboard and CPU as well.
You can get a PCIE card for an NVMe M.2 drive if you want. You can read this article about how the PCIE 2.0 speed will compare to a PCIE 3.0 slot. Sequential read/writes are definitely faster with PCIE 3.0, but with other types of reads/writes, not so much difference. Both are a lot faster than the PCIE SSD compared in that article.

Quote:
Question 2: My motherboard furthermore provides 4 PCIe 2.0 lanes (Not direct to CPU, but rather over the Z68 chipset) ?
The Z68 chipset on your motherboard provides up to 8 PCIE 2.0 lanes. It appears from your motherboard doc that the PCIe x16_3, PCIe x1_1, PCIe x1_2, USB3_34 and eSATA ports all share bandwidth which likely means they are all supported by the Z68 chipset which means they all connect to the processor through the 20GB/s DMI channel (so no direct connection themselves to the processor, but connect through the DMI channel).

Quote:
Question 3a: Where does the 16Gbytes/s come from on the block diagram <<see link above>> on the x16 slot ? I thought PCIe 2.0 offers 500 MB/s per lane (= 8.0 Gbytes/s for x16) ?? <<see PCIe speeds link above>>
Your processor isn't supporting all the direct connection PCIe lanes communicating at full speed at once (which wouldn't actually happen very often in the real world anyway). So, the total bandwidth available is less than sum of the max bandwidth each lane is capable of.

It's kind of like connecting 10 PCs to a router each with a 100Mbps connection and the router has a 500Mbps connection to the internet. Each PC individually could transfer data to the internet at 100Mbps, but all 10 couldn't do it at once. If they were all trying to do max transfer at once, they'd end up sharing the 500MBps connection to the internet and each running about half speed.

Quote:
Question 3b: Same question as question 3a. If i assume the lower 'b' here in 5 Gb/s stand for bits (does the 'Gb' stand for Gbits/s and the GB for Gbytes/s ??), this would make sense since 5 Gbits/s = 625 Mbytes/s (500 Mbytes/s if I take the 8b/10b decoding into account). And 500 Mb/s is what I find to be the speed of PCIe 2.0 per lane in the << PCIe speeds link above >>.
That is the usual convention for Gb/s and GB/s.

Quote:
Question 4: Does my motherboard support NVMe protocol ? OR only AHCI protocol ? (I assume it doesn't support NVMe since it is way old)
No, it doesn't support NVMe directly. That's what you would get the PCIe add-in card for. I don't think NVMe was even around yet when your motherboard was manufactured.

Quote:
Question 5: I read a lot about the fact that PCIe slots can be connected to the CPU using PCIe lanes OR SATA III connection. It depends on the MB manufacterer. Is this true and how can I check this ? My MB manual doesn't specify it.
That's not really the correct way to think of it. PCIe slots can be connected directly to the CPU or they can be connected to some other chipset that communicates with the CPU a different way. In the case of your motherboard, there are 16 lanes of direct PCIe connection to the CPU and the Z68 chipset offers 8 more lanes that connect to the CPU through the DMI channel (all sharing that channel).

SATA ports are also connected through the Z68 chipset and also share the DMI channel to the CPU. The Z68 chipset supports 2 6.0Gb/s SATA ports and 4 3.0Gb/s SATA ports. The other two 6.0Gb/s SATA ports on your motherboard are from another added chipset (A Marvell PCIe SATA controller). From the notes in your motherboard, it appears these are sharing PCIe lanes with some other devices (meaning when one is in use, the other device is disabled).

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2x8GB [email protected] at 1.45V, G.Skill F4-3733C17Q-32GTZKK (XMP rated [email protected])
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post #3 of 7 (permalink) Old 03-02-2019, 02:37 AM - Thread Starter
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Quote: Originally Posted by jfriend00 View Post
The Z68 chipset on your motherboard provides up to 8 PCIE 2.0 lanes. It appears from your motherboard doc that the PCIe x16_3, PCIe x1_1, PCIe x1_2, USB3_34 and eSATA ports all share bandwidth which likely means they are all supported by the Z68 chipset which means they all connect to the processor through the 20GB/s DMI channel (so no direct connection themselves to the processor, but connect through the DMI channel).
I agree with everything you say except that I think the chipset only has 4 PCIe 2.0 lanes. Look at this snapshot from the MB manual:
I have 5 devices that share an x4 connection. If you want the 3th PCIe x16 slot to function on x4 speed, the rest is disabled.. Isn't that proof of only having 4 lanes ?





Quote: Originally Posted by jfriend00 View Post
Your processor isn't supporting all the direct connection PCIe lanes communicating at full speed at once (which wouldn't actually happen very often in the real world anyway). So, the total bandwidth available is less than sum of the max bandwidth each lane is capable of.

It's kind of like connecting 10 PCs to a router each with a 100Mbps connection and the router has a 500Mbps connection to the internet. Each PC individually could transfer data to the internet at 100Mbps, but all 10 couldn't do it at once. If they were all trying to do max transfer at once, they'd end up sharing the 500MBps connection to the internet and each running about half speed.

Good brakedown on the subject, I learned something here ! Except I think the situation here is opposite to what you are explaining? I have 16 PCIe 2.0 lanes (which give max. 8 GBytes/s bandwith all at once. The diagram says the shared connection is 16 GB/s... So why would they give the shared connection twice the bandwidth of what the individual lanes can give all at once ? Seems overkill or a mistake in the manual.





Quote: Originally Posted by jfriend00 View Post
You can get a PCIE card for an NVMe M.2 drive if you want. You can read this article about how the PCIE 2.0 speed will compare to a PCIE 3.0 slot. Sequential read/writes are definitely faster with PCIE 3.0, but with other types of reads/writes, not so much difference. Both are a lot faster than the PCIE SSD compared in that article.


No, it doesn't support NVMe directly. That's what you would get the PCIe add-in card for. I don't think NVMe was even around yet when your motherboard was manufactured.

Good article, I was wondering that too ! The PCIe 2.0 to M.2 adapter card solves the physical part of being able to connect an M.2 SSD in my older MB. But does that add the NVMe protocol to my motherboard as well ?? I don't think that would work, not even if I'de choose to use it as mere storage rather than a bootable storage.





Quote: Originally Posted by jfriend00 View Post
SATA ports are also connected through the Z68 chipset and also share the DMI channel to the CPU. The Z68 chipset supports 2 6.0Gb/s SATA ports and 4 3.0Gb/s SATA ports. The other two 6.0Gb/s SATA ports on your motherboard are from another added chipset (A Marvell PCIe SATA controller). From the notes in your motherboard, it appears these are sharing PCIe lanes with some other devices (meaning when one is in use, the other device is disabled).

I never understood the whole Marvell controller and Jmicron controller that's included on my motherboard. Thank you for making it a bit clearer.
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post #4 of 7 (permalink) Old 03-02-2019, 09:26 AM
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Quote: Originally Posted by 2Dex View Post
I agree with everything you say except that I think the chipset only has 4 PCIe 2.0 lanes. Look at this snapshot from the MB manual:
I have 5 devices that share an x4 connection. If you want the 3th PCIe x16 slot to function on x4 speed, the rest is disabled.. Isn't that proof of only having 4 lanes ?
If you look at this chipset diagram (from the original post in this thread), it sure indicates 8 lanes in the chipset. That's what I was going by. I can't say I fully follow how the sharing works. I wonder if the other PCI lanes are used by the Marvell and JMicron controllers?


[email protected] (-1 AVX offset) on ASRock Z390 Taichi with Noctua NH-D15 air cooler
CPU offset voltage of -25mv, runs VRVout 1.240-1.313V on full AVX load, 1.225-1.275V on non-AVX load
2x8GB [email protected] at 1.45V, G.Skill F4-3733C17Q-32GTZKK (XMP rated [email protected])
EVGA GTX 1060 6GB OC with Corsair RMx 750W power supply
Samsung 970 EVO 500GB NVMe boot SSD and four other drives all in a Fractal Design R6 Case
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post #5 of 7 (permalink) Old 03-02-2019, 10:48 AM - Thread Starter
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Maybe a senior member can elaborate ?
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post #6 of 7 (permalink) Old 03-02-2019, 11:43 AM
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I understand things the same as @jfriend00 . Four of the PCH lanes are used by it to connect to devices on the motherboard and then there's only four left over for use with the PCIe expansion slots.

My Z77 board here has the same setup as what you are seeing. It can do one x4 PCIe slot or several x1 slots. The documentation says if I would install an x4 PCIe card in the full-length slot of the board that's connected to the PCH, then the rest of the slots all become unavailable.
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post #7 of 7 (permalink) Old 03-10-2019, 11:28 AM - Thread Starter
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Some of the answers or doubts were answered on this forum:
https://forums.anandtech.com/threads...nners.2561956/


Thanks everyone for helping me out !
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