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Question PCIEx8 SAS card into a x4 slot?

smegheed

Junior Member
Feb 14, 2020
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Hi, I'm trying to understand a specific thing in regards to plugging a PCIEX8 adapter card into a PCIEX16 slot that runs in x4 mode and only has the pins for a x4 card.

This is the card I'm using: MegaRAID LSI SAS 9260-8i KIT 8-Port 6Gbps SAS/SATA 512MB PCI-E 2.0 x8

This is the Motherboard I'm using: GA-Z77X-D3H

Here are its PCIE specs:

1. 1 x PCI Express x16 slot, running at x16 (PCIEX16)
* For optimum performance, if only one PCI Express graphics card is to be installed, be sure to install it in the PCIEX16 slot.

2. 1 x PCI Express x16 slot, running at x8 (PCIEX8)
* The PCIEX8 slot shares bandwidth with the PCIEX16 slot. When the PCIEX8 slot is populated, the PCIEX16 slot will operate at up to x8 mode.

3. 1 x PCI Express x16 slot, running at x4 (PCIEX4)
* The PCIEX4 slot shares bandwidth with the PCIEX1_1/2/3 slots. The PCIEX1_1/2/3 slots will become unavailable when a PCIe x4 expansion card is installed.

4. 3 x PCI Express x1 slots
(PCIEX4 and PCIEX1 slots conform to PCI Express 2.0 standard.)

The questions i have are:

If I plug the SAS card into the second PCIEX16slot running at x8 along with a graphics card in the first x16 will it affect the performance/create issues for either card?

From what u I've read I understand doing this with just 2 graphics cards isn't a problem and neither saturates the bandwidth available but I'm not sure about a SAS card. Will this create issues like little freezes/stutters when doing something graphic intensive while accessing hard drives via the SAS card, or some other problem?

My preference is to just put the SAS card into the 3rd PCIEX16 slot and leave the graphics card in the first so they are completely separate and not sharing anything.

However I then have the question of is running a x8 SAS card in a x16 slot but only running in x4 mode with the pins of a x4 slot a bad idea/worse than using the 2 8x slots?

What has me particularly worried about using the x4 slot is I can see how the number of physical pins the slot has to connect with the SAS card is less then what the other 2 slots have.

Does putting a card that has x8 pins into a slot with only x4 pins in it simply reduce its performance or will it cause problems if used like this?

Looking at graphics card as an example it seems it's ok to put a x16 card into a x8 slot even though the number of pins is reduces in the second x16/x8 slot. So I assume putting an 8x card into a x16/x4 slot with fewer available pins is also ok?

If so I'm confused as to how a card that has 16x or 8x pins on it still works in a slot without all those pins making contact? If it works with less pins why does it have them all to start with? And why does it work when there are less pins anyway?

Any help appreciated.
 

Topweasel

Diamond Member
Oct 19, 2000
5,240
1,371
136
Can't answer or maybe just unwilling to answer all 20 questions. But Ill go with the first one. A video card specially one over 4GB in size will see almost no loss in performance on 8x vs. 16x. So the question is will the Sas card lose performance on 8x vs. 16x. The answer being no, its a 8x card. So second PCIe 16x is probably the best solution.

Those aren't shared. They are split. So 16x to Slot 1 if nothing in slot 2. If something in slot two then it sends 8x distinct lanes to slot 1, 8x distinct lanes to slot 2.
 

Topweasel

Diamond Member
Oct 19, 2000
5,240
1,371
136
Also with the less. The idea is bandwidth and power. Some things just need the extra power. The other is to allow variance for bandwidth, almost always a device needs less then the bandwidth the connector allows for, gives it room to breathe.

That said When dealing with SSD's you can almost always find situations where the drives fill up the bandwith. So whatever the card is capable of I would make sure it gets it.
 
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