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Performance Estimates and Results
USB 3.0 in Super Speed mode offers a maximum potential data throughput of 500 MB/sec after allowing for 8b/10b encoding, but various bus overheads detract from this maximum in actual operation. It has been estimated (as explained in the UASP standard) that an actual sustained data throughput of 400 MB/sec may be attainable using UASP, compared to an estimated limit of only 250 MB/sec for BOT operation on a USB 3.0 link without UASP.
Actual test results using the Renesas UASP driver and Renesas µPD720230 USB3-to-SATA3 bridge device are shown in Figure 1. The testing was performed using the CrystalDiskMark3 benchmark program, running under Windows 7 (64-bit) on a PC having a PCIe bus running at 5.0 GHz, with a Renesas µPD720201 USB 3.0 Host Controller and Renesas USB 3.0 (xHCI) driver for the host controller. The mass storage device was a Crucial RealSSD solid-state drive with a Renesas µPD720230 USB3-to-SATA3 bridge. In this test, both sequential read and random read with 512KB block size performed significantly better than 250 MB/sec in BOT mode, and up to 369.2 MB/sec with UASP enabled. Although the UASP results didn't quite achieve 400 MB/sec, UASP still provided up to 30% performance improvement over BOT. Gains for writes to the drive were more modest: 2.7% for sequential writes, and 1.9% for 512K random writes.
Test results can vary widely, however, depending on the specific benchmark program used, its configuration, and the capabilities and configuration of the various hardware elements beyond the SATA bridge and USB3 link.
Figure 1: Benchmark Software test results (MB/s) shows how UASP improves both sequential and random access to USB storage devices.