Trying to understand slow down in file copies over TB3 on Mac Pro 2019

[Pyronaut]

I've been doing various file transfers over Thunderbolt 3 on a 2019 Mac Pro (with 8 TB3 ports) and noticed some perplexing results that I'm trying to understand.

I have a very fast (10Gb/s) memory card in a TB3 card reader that I'm copying to an 8TB NVMe drive in a TB3 enclosure. If I'm doing one card at a time, 80GB of data finishes in about 3 minutes using ShotPut Pro, which copies the files and then does a read pass to verify the checksum using XXHash64.

If I do the same transfer again but also concurrently copy from a second fast (10Gb/s) card with roughly the same amount of footage using a separate card reader (going to the same location), they both finish in about 6 minutes. That's reasonable since 3+3=6. If I copy the same amount of footage from a slow (1.5GB/s) card using the same fast TB3 card reader as the previous run, it takes just under 11 minutes. Again, reasonable since the card has a much slower read speed.

What's weird is that if I initiate a copy from the same fast card as the previous run while the slow card is copying to the same location, that transfer gets slowed way down and they both finish in about the same amount of time (around 10.5 minutes).

It seems as though the system is transferring both cards at the same speed, even though one card should be able to go much faster than the other. All three devices - the two card reader sources and the NVMe drive destination - are on different Thunderbolt 3 buses so I don't think that they should slow each other down in the way that I'm seeing.

Does anyone have any idea on why this might be happening? I don't understand why two fast cards would copy quickly at the same time, but a fast card and a slow card would slow down the fast card to the slow card's speed.

 

[Lufbrarunner]

Just thinking out loud, it seems the fastest the operation will work at is the speed of the slowest component. So the slow card is the limiting factor.

 

[Pyronaut]

Just thinking out loud, it seems the fastest the operation will work at is the speed of the slowest component. So the slow card is the limiting factor.

Yeah that looks to be the case, but what I'd like to know is why. The slow card is in a different card reader and on a different Thunderbolt bus, so it doesn't make sense to me that it would slow down the transfer of the faster one.

 

[Pyronaut]

Finally figured it out. It's an issue with ShotPut Pro. It tries to make the transfers go around the same speed to reduce thrashing, but it's a waste for NVMe drives.