Thunderbolt: How Devices Affect One Another on a Daisy Chain

The Macworld Lab has a small collection of Thunderbolt peripherals, and we thought we should put them to work to answer questions we had about connectivity and how devices affect performance.
After testing dozens of scenarios, we found that—for the most part—the two available bi-directional 10Gbps channels in the MacBook Pro (Late 2011) were more than able to keep up with the demand of multiple storage devices on a Thunderbolt daisy chain. However, if you add multiple displays to that chain, the throughput of some drives can be severely limited.
(For your reference: The iMac (Mid 2011) uses the same Light Ridge Thunderbolt controller as the MacBook Pro, but the iMac has two Thunderbolt ports and offers up to four bi-directional, 10Gbps channels. The MacBook Air (Mid 2011) uses an Eagle Peak Thunderbolt controller that allows for two bi-directional 10Gbps channels.)

Drives in a daisy chain

The Thunderbolt drives involved in our testing were the Promise Pegasus R6 RAID array, the LaCie Little Big Disk Thunderbolt Series SSD, and the LaCie Little Big Disk Thunderbolt Series 2TB hard drive.
To get a set of baseline speeds to gauge performance, we attached one drive at a time, with no other devices, to a 17-inch 2.4GHz Core i7 MacBook Pro. We ran the AJA System Test several times—specifically, the 2GB, 1920-by-1080, 10-bit RGB test—and calculated average performance speeds. The Pegasus R6 was the fastest device we tested, with a write speed of 538.1MBps and a read speed of 519.3MBps. The LaCie SSD posted a write speed of 253.0MBps and a read speed of 480.5MBps. The LaCie hard drive clocked in with a write speed of 184.7MBps and a read speed of 203.1MBps.
We then attached all three storage devices to the MacBook Pro, with the Pegasus R6 first in the chain connected directly to the MacBook Pro. The LaCie SSD was second in the chain, and the LaCie hard drive was last. When we ran the AJA System Test again, the Pegasus R6 saw a slight performance improvement, with a write speed of 543.0MBps, and a read speed of 520.3MBps.
The LaCie SSD, second in the chain (attached to the Pegasus R6), scored about the same as it did when connected alone, with a write speed of 246.9MBps and a read speed of 469.4MBps. The LaCie hard drive, the last device on the chain (attached to the LaCie SSD), posted scores that were a touch faster than when attached alone: a write speed of 190.1MBps, and a read speed of 205.4MBps.
We then changed the order of the devices on the chain, placing the Pegasus R6 last on the chain, and the LaCie SSD first, with the LaCie hard drive in-between. The Pegasus R6 scores were just a touch slower, with a write speed of 532.8MBps, and a read speed of 513.2MBps. With the LaCie SSD connected to the MacBook Pro, it scored about the same as it did in the second position. The same was true for the LaCie hard drive.

Results are in MBps. Testing performed on a late 2011 17-inch 2.4GHz Core i7 MacBook Pro.—Macworld Lab testing by James Galbraith and Mauricio Grijalva

Drives and a display in a daisy chain

In our next series of tests, we added an Apple Thunderbolt Display to the end of the chain. All of the drives turned in speeds very similar to the speeds they posted without the display. Swapping the Thunderbolt Display with an Apple 27-inch LED Cinema Display didn’t change performance much, either.
With the Thunderbolt Display as the first device (and the only display) in the chain, the LaCie SSD experienced a small performance hit. There was no real performance change on the LaCie hard drive.

pcworld

Posted in: Apple News