Several questions about GPU mining - R9 280x

[psijic]

Hey everyone. I'm about to build my first mining rig this week, and have several hardware related questions.

1. Is anyone currently using multiple MSI R9 280x cards with ASRock 970 Extreme 4 motherboard? I've read that they should be compatible, but I've also read some reports about people having issues with compatibility.

2. Most motherboards which have multiple PCI-e slots, provide decreasing speeds like 1 slot- x16, 1 slot- x8, 1 slot- x4 etc. Does this impact mining performance? I have yet to come across any motherboard which provides x16 speed on all slots.

3. How safe is flashing a GPU's BIOS? Apparently, its required for under-volting cards when working on linux, but I'm concerned about the possibility of killing the cards.

4. Has anyone ever used Add2PSU (http://www.add2psu.com/) to connect multiple PSUs in one rig? Are these things safe to use? A 1200W PSU is very expensive, and may not support 4 to 5 cards, however, connecting two 860W PSUs can definitely solve the problem.

[GoldSeal]

I use that board with 7970's. It shouldn't be a problem. Just make sure you use flexible cable risers with a a separate 12v molex. You don't want to draw power through the motherboard if you will be running more than 2 cards. Consumer grade boards are not made to deliver 75 watts per card through the pcie interface. FOr a genric 300 watt card you should get approximately 225 watts in from the pcie plugs and the other 75 watts from the 12v line on the flexible riser.

With regard to decreasing speeds and slot type, it doesn't matter. 1x pcie is all that's needed for mining with any gpu. You can plug a 1x -> 16x cable riser into any pcie slot and it will function fine.

firmware: These days, it's not too risky unless you lose power. However, you always run the risk of bricking a card and may have to rma it. SO have a good reason to flash. DOn't just do it because you think the newest firmware is the best.

I tried one of the add2psu modules with 2x 1600 watt psu's. It was a no go for me. One psu registered an error led when it was connected. The add2psu device isn't compatible with alot of higher end psu's that have advanced fault protection features. Best advice I can give you is to avoid that gimmicky stuff.

[psijic]

I use that board with 7970's. It shouldn't be a problem. Just make sure you use flexible cable risers with a a separate 12v molex. You don't want to draw power through the motherboard if you will be running more than 2 cards. Consumer grade boards are not made to deliver 75 watts per card through the pcie interface. FOr a genric 300 watt card you should get approximately 225 watts in from the pcie plugs and the other 75 watts from the 12v line on the flexible riser.

With regard to decreasing speeds and slot type, it doesn't matter. 1x pcie is all that's needed for mining with any gpu. You can plug a 1x -> 16x cable riser into any pcie slot and it will function fine.

firmware: These days, it's not too risky unless you lose power. However, you always run the risk of bricking a card and may have to rma it. SO have a good reason to flash. DOn't just do it because you think the newest firmware is the best.

I tried one of the add2psu modules with 2x 1600 watt psu's. It was a no go for me. One psu registered an error led when it was connected. The add2psu device isn't compatible with alot of higher end psu's that have advanced fault protection features. Best advice I can give you is to avoid that gimmicky stuff.

Thanks for the answers. I have a few followup questions.

1. Is there any difference between 280x cards from different brands? Can I expect the same performance / reliability / stability from 280x cards by different companies?

2. Can I use multiple 280x from different brands in one rig? If yes, will they need to be configured separately, for all of them to reach max potential?

3. Which brand's 1600W PSU do you use? Also, how do people use 5 or even 6 GPUs in their rigs with a single power supply?

4. What difference can I expect if I decide to go with Windows 7 instead of Linux, as the OS in my rig? I really want to stick to Linux, but most brands have Windows based tools to under-volt / over-clock cards without having to mess with the BIOS.