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OWC Thunderbay 6: RAID-0 Performance

OWC Thunderbay 6

Configurations up to 84TB. Available in RAID edition (includes SoftRAID XT) or standard edition.

The OWC Thunderbay 6 houses six hard drives in one enclosure.

Bandwidth

Thunderbolt 3 provides up to about 2700 MB/sec (2.7 GB/sec).

However, each pair of hard drives (A/B and C/D and E/F) in the OWC Thunderbay 6 shares a SATA controller* each on a single PCIe lane (raw bandwidth 500MB/sec). Including parity information and other overhead, the maximum bandwidth for each pair of drives is around 400 MB/sec when parity is accounted for. Thus the total bandwidth to the six hard drives cannot exceed ~1200 MB/sec, but overhead and data parity across the bus reduce that to around ~383 MB/sec, or about 1150 MB/sec.

* The OWC Thunderbay 4 dedicates a SATA controller to each of its four drives (4 controllers). It is unclear why OWC chose to use three SATA controllers in the Thunderbay 6 (instead of six) when 4 controllers are used in the Thunderbay 4. Perhaps because six controllers would have meant using 6 PCIe lanes which exceeds the bandwidth of Thunderbolt 3, whereas 3 lanes for the hard drives plus 2 lanes for the PCIe slot is 5 lanes, within the bandwidth. Still, that would be silly choice, since using hard drives plus a PCIe SSD simultaneously would still fit into the available bandwidth.

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Testing details

Six Toshiba 14TB hard drives were used. These are very fast state of the art hard drives, the fastest MPG has yet tested as of early 2019.

Tested using the run-area-test of diglloydTools Disktester as follows:
disktester run-area-test --iterations 5 --xfer 768M --test-size 8G

Test mule was the 2018 Mac mini. The tests were run with all six drives in a single OWC Thunderbay 6 enclosure, and again with 3 drives in two OWC Thunderbay 6 enclosures, each on a different Thunderbolt 3 bus.

Test results, RAID-0

Like all hard drives, the outer tracks are much faster than the inner tracks; see Drive Capacity and Speed. Hence the speed across capacity steadily declines as expected behavior.

The peak speed with a single OWC Thunderbay 6 enclosure is throttled to about ~1155 MB/sec due to the bandwidth limits of its three SATA controllers. Within a small deviation, the speed is constant out to about 80% of the drive capacities—over a gigabyte per second for 67TB of storage capacity! Write speed runs about 10% slower than read speed in a single Thunderbay 6 enclosure.

Perspective check: even with a single enclosure and nearly 100% of capacity, the RAID-5 speed greatly exceeds the sustained transfer performance of a fast SATA SSD (about 550 MB/sec)!

With two enclosures, the bandwidth limits disappear and the performance potential of the Toshiba 14TB drives is made manifest: about 1440 MB/sec dropping to the same speed as a single enclosure at about the 80% capacity mark (about 67TB). Note also that the read and write speeds using dual enclosures are much closer, and that write speeds are actually higher than read speed with dual enclosures.

While full hard drive performance in a single enclosure would be tasty, there are few scenarios in which over a gigabyte per second over 67TB of capacity is an issue! For most all uses, a more practical thought process is the very high speed over the huge capacity.

* Reality check: given the cost of the 14TB hard drives used here, the cost of a second enclosure (or two Thunderbay 4 enclosures) is nominal, so those seeking the absolute best performance could buy a second enclosure and forget about the issue.

Vertical scale is MB/sec. Horizontal scale shows percentage of volume capacity, e.g., 50% of an 84TB RAID-0 stripe is 42TB of the capacity.

OWC Thunderbay 6: RAID-0 performance using 6 drives in one enclosure versus two enclosures
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