USB attached SCSI

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The USB Attached SCSI or USB Attached SCSI Protocol , abbreviated to UAS or UASP , is a communication protocol for the connection of mass storage devices such as hard disk drives , solid state drives (SSD) or DVD drives with SCSI or Serial ATA connection via the Universal Serial Bus (USB) to a computer system. UAS was introduced in the course of the USB 3.0 standard and, with a reduced data transfer rate , is backwards compatible with the older USB 2.0 standard. The older and slower alternative is called USB Mass Storage Bulk-Only Transport ( BOT ).

With UASP it is possible to achieve a particularly fast data transfer of approx. 450 MB / s (= 3,600 Mbit / s) with an SSD or a USB memory stick that is connected via USB 3.0 or higher. Provided that all parts of the transmission chain (memory controller, chipset of the mainboard and operating system) support UASP; Conventional hard drives, including those with a SCSI interface, are inherently too slow for such transfer speeds. The specific values ​​depend heavily on the hardware used.

In contrast to the Bulk Only Transfer (BOT) used in dedicated USB mass storage devices such as USB memory sticks and which is technically easier to implement , with UAS the control commands of the SATA and SCSI interfaces are routed directly via the USB interface. Among other things, this allows the use of SMART via USB , a method for self-monitoring and predicting failures in mass storage devices. In addition, UAS allows a higher data throughput because, in contrast to BOT, the commands and status queries run via separate data channels. In addition, UAS-compliant devices can process several commands in parallel, also known as Native Command Queuing (NCQ).

The support of UAS depends not only on the hardware but also on the software in the computer system. For example, UAS is only supported from Linux kernel version 3.15. If the computer system does not support the UAS operating mode, the hardware switches to Bulk-Only Transport (BOT) and thus allows access to the mass storage device with a significant reduction in data throughput. Windows 10 and 8 support UASP out of the box; Windows 7 requires special drivers.

Web links

Individual evidence

  1. ↑ a b Curtis E. Stevens: Mass Storage Class - USB Attached SCSI Protocol (UASP). USB Implementers Forum (usb.org), June 24, 2009, accessed October 28, 2016 .
  2. ↑ Manuel Masiero, Achim Roos: USB 3.0 afterburner UAS. February 28, 2013, accessed March 8, 2019 .
  3. ↑ Mass Storage Class - Bulk-Only Transport (BOT). (PDF) USB Implementers Forum (usb.org), September 30, 1999, accessed October 28, 2016 .