Thunderbolt: How devices affect each other on a daisy chain

By James Galbraith, Macworld |  Storage, Apple, Thunderbolt

We also ran a few tests to see if the performance degradation would show up in everyday use. We copied a 6GB file from the iMac to the Pegasus R6, duplicated the file on the Pegasus R6, and then copied the duplicate file back to the internal SSD on the iMac. We saw a Pegasus R6 write speed of 225MBps in two configurations: with just a single Thunderbolt Display attached to the array, and then with the Thunderbolt and Cinema Display attached. With two Thunderbolt displays, the write speed slowed down a bit to 214.3MBps. Duplicating the file on the Pegasus R6 showed bigger differences, with 299.0MBps with no display attached, 297.5MBps with a single Thunderbolt Display attached, 247.1MBps with a Thunderbolt Display and a Cinema Display attached, and just 163.8MBps with two Thunderbolt Displays attached. When reading the files back to the internal SSD, we saw less of a performance difference, with speeds ranging from 184.0MBps with no display attached to 178.0MBps with two Thunderbolt Displays connected.

The reason for the performance decrease in a two-display setup is two-fold. First, the Pegasus is fast and can put plenty of bandwidth to use. Second, each Thunderbolt Display gobbles up about 7Gbps of an outbound channel's available 10Gbps bandwidth. Display traffic is one-way, so that's why the read speeds from the Pegasus aren't as affected. To test this out, we attached the second Thunderbolt Display to the iMac's second Thunderbolt port and found performance to come back to the level of a setup with just one Thunderbolt Display attached.

Thunderbolt Software Update 1.1

During our testing, we found that the Pegasus R6 was faster before applying the Thunderbolt Software Update 1.1. The file transfer tests we ran on the iMac were very close to the tests runs with the update applied, but our file duplication tests saw speeds as fast as 341.7MBps without the update, versus a peak of 299.0MBps with the update.

Using the AJA System Test with the iMac, we saw a peak write speed of 644.3MBps and a peak read speed of 521.3MBps from the Pegasus R6. Interestingly, if we restarted the Mac with the Pegasus R6 connected, but did not restart the array, write speeds would drop to about 559.9MBps. Read speeds weren't affected much, with speeds averaging 525.4MBps. Restarting the Pegasus R6 would bring the write speeds back up to the 644MBps range.

Once we installed Thunderbolt Software Update 1.1, the scores stayed the same whether we restarted or not. The Pegasus R6's write speed of 534MBps, however, was a drop from the 644MBps we saw before installing the update. The Pegasus R6 had a read speed of 520MBps. Neither of the LaCie drives were affected by the update.


Originally published on Macworld |  Click here to read the original story.
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