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OWC 30 Meter Optical Thunderbolt Cable: Does Length Matter for Performance?

Thunderbolt cables range in length with common sizes of about 1/2 meter, 1/2/3 meters, 10/20/30 meters. View available lengths.

At 10 meters and beyond Thunderbolt cable prices increase greatly because of the use of optical fiber along with a controller that converts the electrical signals to optical pulses.

But with the longer lengths, MPG wondered if there might be a performance impact. due to increased latency (slight delays in signal).

To answer that question, a 4-drive RAID-0 stripe was created using SoftRAID 5 in an OWC Thunderbay enclosure using very fast hard drives that are just about capable of maxing-out the Thunderbolt bandwidth.

RAID-0 stripe: 4 very fast hard drives

Testing using DiskTester, with the run-sequential suite command.

RAID-0 striping varies in speed depending on transfer size (overhead, etc), but with a 32K stripe size, the starting I/O transfer size of 128K allows all four drives to participate. As can be seen in the graph, throughput rises with larger transfer size regardless of cable used.

Read speed is about 8% faster with the 1 meter Thunderbolt cable (green line) than the 30 meter optical cable (blue line). For really huge transfer sizes (10MB and larger) there is effectively no difference, but such large transfers would be rare; few programs would make such large I/O requests. For example, a program like Photoshop uses up to 1MB transfers, and for 1MB transfers a 7-8% difference is seen.

Not explored is the impact with small transfer sizes to a fast SSD or single hard drive. The results suggest that latency is the issue, hence smaller transfers (less than 128K) might be more affected.

Bottom line: a shorter cable offers higher performance. But if the goal is to displace the storage solution (e.g. for noise reasons), then a longer cable is a win. And these modest speed differences are unlikely to matter for many if not most applications.

Transfer speed over range of I/O sizes, 1 meter vs 30 meter cable

RAID-0 stripe: 2 OWC Mercury Extreme Pro 6G SSDs

Testing using DiskTester, with the run-sequential suite command.

The SSDs behave consistently with the hard drives but the effect is muted. Smaller transfers are just a little faster with the 1 meter cable, and large transfers are so close in speed to be effectively identical.

Transfer speed over range of I/O sizes, 1 meter vs 30 meter cable


MacPerformanceGuide.com
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