Thunderbolt 3 - One Bus Performance?

[Redneck1089]

I am considering purchasing a new iMac for professional photography and videography usage. I currently have a Dell 5K screen that I will want to use in conjunction with the new iMac.

This presents an obvious problem -- namely, that the Dell 5K uses two display ports and the iMac only has two Thunderbolt 3 ports on a single bus.

I would want to run an external GPU via an Akitio Node with a GTX 980 Ti Superclocked, which would then have the Dell 5K plugged into the Nvidia card.

That then gives me back one Thunderbolt 3 port.


My question is: since the iMac has only one Thunderbolt 3 bus, unlike the two on the MacBook Pro and upcoming iMac Pro, how well would the existing Thunderbolt bus holdup with an additional RAID Thunderbolt 3 SSD/HDD enclosure plugged in along with the eGPU with Dell 5K?

I'm worried that the bus might be oversaturated, which then leads me to think I should look at the iMac Pro.




Thanks in advance for your thoughts.

 

[satinsilverem2]

I dont see it being an issue most of the time unless you are rendering and using the RAID at the same time. The only real solution would be convert the RAID array to a USB C Gen 2 enclosure as that would use the USB bus and not the Thunderbolt bus. you would still get respectable speeds from the RAID over type C Gen 2

 

[Redneck1089]

I dont see it being an issue most of the time unless you are rendering and using the RAID at the same time. The only real solution would be convert the RAID array to a USB C Gen 2 enclosure as that would use the USB bus and not the Thunderbolt bus. you would still get respectable speeds from the RAID over type C Gen 2

That's great. Thank you for the information.

 

[whwang]

One thing related is TB3 external enclosures. Those from Akitio can only achieve something like 1500 MB/s, even with 4 SSD inside in RAID0. This is much lower than TB3's bandwidth. Akitio said this has something to do with the PCIe to SATA conversion, and this may apply to other brands' external enclosures too.

If the above is true, your TB3 external enclosure won't saturate the bandwidth.

 

[Redneck1089]

One thing related is TB3 external enclosures. Those from Akitio can only achieve something like 1500 MB/s, even with 4 SSD inside in RAID0. This is much lower than TB3's bandwidth. Akitio said this has something to do with the PCIe to SATA conversion, and this may apply to other brands' external enclosures too.

If the above is true, your TB3 external enclosure won't saturate the bandwidth.

Interesting information. Thank you!

 

[Chancha]

I am thinking that since the eGPU side of the ports has the job of compute offloaded to the 980Ti anyway, the most demanding bandwidth on the chain is then only happening between the card and the Dell 5K via MST DP. I am not entirely sure on the behaviour of the 2 TB3 ports sharing the same controller, but even if there is only one port's full bandwidth/lanes available, a 40Gbps in total should be plenty enough for your particular setup. And then video related tasks that demand GPU power can be offloaded to only the internal dGPU where no lanes are required from the TB3 bus.

BTW, IMO running dual 5K screen is quite unnecessary and it is as you can see, tricky to setup for now. I personally would be much more interested in a single DCI-4K proofing monitor with HDR10 and DCI-P3/ARGB, which is doable by just one DP1.4 connection. But I do understand that the Dell 5K is still quite a good monitor and of course you would want to make full use of it, despite the dual MST requirement being quite a huge hassle to deal with.