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Is there a noticable difference between OB SATA bandwidth vs PCI-e SATA Controller

Is there a noticable difference between OB SATA bandwidth vs PCI-e SATA Controller

Main Question : Would I see any benefit (I/O bandwidth-wise) to purchasing a separate (non-raid) PCI-e controller card to plug SATAII drives into vs on-board SATAII slots?

Asked by: Guest | Views: 17
Total answers/comments: 2
Guest [Entry]

No, you won't see any benefit. In fact there might be a very slight performance decrease if you use the PCI-e SATA controller.
Guest [Entry]

"Is there a noticable difference between OB SATA bandwidth vs PCI-e SATA Controller?

tl;dr: rarely but it is possible. Onboard devices have only 2 or 4 GByte/s bandwidth which you can saturate while some PCIe slots are directly CPU attached with much more bandwidth.

On board SATA on Intel motherboards these days are fed from the chipset which is connected to the CPU by the DMI bus. DMI 3.0 has the same bandwidth as PCIe 3.0 x4 just a hair below 4GByte/s. Not only the SATA devices are fed from this connection but everything else on the MB as well, including M.2 (max 4 GByte/s), USB 3.1 Gen 1 (5 Gbit/s) and Gen 2 (10 Gbit/s) controllers, and some PCI Express slots as customer Intel CPUs only have 16 PCI Express lanes on the CPU, the rest is supplied by the chipset.

Now you can see where the trouble is and motherboard makers know this as well because using a single M.2 slot typically disables two SATA ports. The M.2 slot itself uses at most one SATA lane but the trouble is when that's not used because you are using a PCIe based M.2 drive which singlehandedly eats enough bandwidth to consume the bandwidth of the entire DMI bus.

So on an LGA 1151 based consumer board if

A PCIe M.2 SSD is used
The SATA controller is in a CPU-fed PCI Express slot (your motherboard manual will tell you which one it is, sometimes you can configure your slots to be like x8 + x8 or x8 + x8 + x4)

then the SATA controller will be faster. Otherwise, you are fighting for the same DMI bus bandwidth so it doesn't matter. Do note, however, this is mostly theoretical, even the fastest SSDs currently do not eat more than 2.5GByte/s so unless you try to do something like copying from a Samsung 960 Pro to four SATA SSDs in RAID-0 you won't hit the bandwidth cap.

On an LGA 2011v3 motherboard the situation is even more interesting. The X99 chipset only supports DMI 2.0 which is half the speed, that is 2GByte/s and since a single plain SATA disk can consume .5GByte/s if you want to go above four SATA disks then going with a PCI Express controller will be faster because on these motherboards typically two x16 and one x8 slot (or in rarer cases, five x8) is fed directly from the CPU as most LGA 2011v3 chips except for the 6800K and the 5820K have 40 PCI Express lanes.

Finally, if you are using an older board with Z97 or older chipset which only supports DMI 2.0 at 2GB/s then again a single M.2 slot (some Z97 boards had M.2 slots) or four SATA disks will consume the available bandwidth and then the CPU fed slot card will be faster again."