Thunderbolt / eSata rMBP data pipeline question

[Shane Daly DP]

Hi all

I have a Magma chassis running my Rocket to my Retina MBP, I am discussing with my DIT streamlining our on set work flow.

AFAIK the Thunderbolt connection to the Magma is fully used by the Rocket running at 50% capacity at x4.

Therefore would it be a poor decision to add an eSata card to the Magma as transferring data would then slow the Rocket?

The other option would be to run the Lacie Thunderbolt / eSata box via the 2nd Thunderbolt connection.

Our research has proved inconclusive. The pertinent question really is whether the two Thunderbolt connections on the rMBP are actually two distinct, separate routes or whether they actually run into the same bus?

Any concrete, definitive advice gratefully received.

Thanks all.

Shane
shanedaly.info

 

[Jeff Kilgroe]

The two thunderbolt ports on the system are two distinct ports. That is a requirement in the Thunderbolt spec, they don't get shared as USB and Firewire usually do.

Also remember that each Thunderbolt port has not one, but two 10Gbps data channels. You can connect the LaCie eSATA box in a daisy-chain arrangement on the same port as your Magma box and it will run on a different 10Gbps channel than the Magma!

There is one catch to all of this. While each TB port is distinct, they are each backed by a PCIe v2.0 X4 host. In a nutshell, that means each port is capable of 20Gbps, but only backed by 16Gbps of throughput. In most situations that is not an issue because you're not going to saturate both channels all the time.

Adding an eSATA card to the Magma box would indeed cut into the Rocket's bandwidth, but only if you're transferring significant amounts of data via that eSATA card while the Rocket was churning out imagery. All the cards in the Magma (or whatever PCIe expander you're using) must share the 10Gbps of available data bandwidth on whichever TB channel the expander is connected. If you daisy-chain more than two devices, then the two channels have to start being shared amongst other devices in the chain.

The Magma has an X8 slot in which to install the Rocket. So, it will allow access to all of the Rocket's PCIe lane channels. But even if it's the only card in the Magma box, all of it's data has to fit through that 10Gbps of available throughput. Effectively restricting the Rocket to half it's potential speed. Not a problem in most situations with 4K 24fps and lower. However working in 5K will bottleneck at the TB interface. It will also bottleneck ad other places on this system in 4K and 5K. One of the biggest shortcomings of the MBPr is the 16GB RAM. Apple did an awesome job with the system, but it really desperately needs the option to have 32GB.