2020 iMac 27” Target Disk Mode Performance

[jasonefmonk]

I’ve had more than one iMac delivered due to unrelated issues, so I had the opportunity to move a large amount of data from one Mid-2020 27-inch iMac to another. I wanted to see what was possible using Thunderbolt 3 and Target Disk Mode, but it hasn’t met my expectations yet.

Currently moving 6TB of data from the 8TB SSD to the 8TB SSD via the Apple 0.8m Thunderbolt 3 cable; averaging ~360MBps read and write.

Where is the bottleneck? Blackmagic Disk Speed Test measures those drives at R: >2,600MBps, W: >2,900MBps. Thunderbolt 3 is supposed to have bandwidth for 5,000MBps.

Is it something about TDM that makes the Mac not perform at peak? Do I need an active Thunderbolt cable? Am I misreading some numbers along the way?

The one iMac is lying face down, but seems to be cooling fine. I know that isn’t within normal operating tolerances but it couldn’t be avoided because they are VESA models and I don’t have multiple mounts.

Thanks in advance for your insight.

 

[jasonefmonk]

Is it something about TDM that makes the Mac not perform at peak?

Maybe I've found the bottleneck. When the source iMac is in Target Disk Mode it is running EFI and isn't using all the macOS bells and whistles to get full bandwidth from Thunderbolt 3.

A solution could be to use Thunderbolt Bridge network interface, but I won't be able to test. It seems simple enough to set up however: https://sixcolors.com/post/2015/04/speedy-transfers-with-thunderbolt-over-ip/

 

[cltd]

Target disk mode is an emergency solution, so I wouldn't expect full performance but would be happy that works