USB Attached SCSI
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USB Attached SCSI (UAS) is a computer protocol used to move data to and from USB storage devices such as hard drives (HDDs), solid-state drives (SSDs), and thumb drives. UAS depends on the USB protocol, and uses the standard SCSI command set. Use of UAS generally provided faster transfers, when compared to the older USB Mass Storage Bulk-Only Transport (BOT) drivers.
UAS was introduced as part of the USB 3.0 standard, but can be used for slower USB 2.0 devices assuming compatible hardware, firmware and drivers are used.
UAS is defined across two standards, the T10 "USB Attached SCSI" (T10/2095-D) referred to as the "UAS" specification, and the USB "Universal Serial Bus Mass Storage Class - USB Attached SCSI Protocol (UASP)" specification. The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the UAS specification; the SCSI Trade Association (SCSITA) promotes the UAS technology. The USB mass-storage device class (MSC) Working Group develops and maintains the UASP specification; the USB Implementers Forum, Inc. (USB-IF) promotes the UASP technology.
UAS drivers generally provide faster transfers when compared to the older USB Mass Storage Bulk-Only Transport (BOT) protocol drivers. Although UAS was added in the USB 3.0 standard, it can also be used at USB 2.0 speeds, assuming compatible hardware.
A brief hardware roundup in July 2010 by SemiAccurate found that Gigabyte Technology had introduced working UAS drivers for their boards using NEC/Renesas chips, and that—on the hardware level at least—"the LucidPort USB 300 and USB302, Symwave SW6315, Texas Instruments TUSB9260 and the VLI VL700 controllers all support UASP, while as far as we could find, the ASMedia ASM1051 and ASM1051E as well as the Fujitsu MB86C30A doesn’t."
A comparative performance review by VR-Zone in August 2011, concluded that only the NEC/Renesas chips had UAS working drivers. The same Renesas UAS driver (for Windows) also works with AMD's A70M and A75 Fusion Controller Hubs, the USB part of which was co-developed by AMD and Renesas. In October 2011, ASMedia chips had gained driver support as well (they had support on the hardware side before). Fujitsu lists some higher-end chips like the MB86C311A that do support UAS.
As for support by Intel Platform Controller Hub (PCH), an article in MyCE notes: "The native Intel USB3 UASP solution is only supported under Windows 8. To further complicate matters, not all Z77 motherboards support USB3 UASP. A license is required to implement UASP, and not all motherboard manufacturers are prepared to pass on the extra cost of this license to the end user."
Operating system support
Microsoft added native support for UAS to Windows 8. Drives supporting UAS load Uaspstor.sys instead of the older Usbstor.sys. Windows 8 supports UAS by default over USB 2.0 as well. UAS drivers and products are certified by Microsoft using the Windows Hardware Certification Kit.
As of 2012[update], the Linux kernel also had native UAS support, but it had compatibility problems with Texas Instruments chipsets. The Linux driver had "broken" status from December 2012 until September 2013. Version 3.18-rc4 of the Linux kernel disables buggy UAS implementation in xHCI host controllers Etron EJ168, ASMedia ASM1042, and VIA VL80x.
- Designed to directly address the failings of the USB mass-storage device class bulk-only transports (BOT)
- Enables command queuing and out-of-order completions for USB mass-storage devices
- Eliminates software overhead for SCSI command phases
- Up to 64K commands may be queued
- SCSI SAM-4 compliant
- USB 3.0 SuperSpeed and USB 2.0 High-Speed versions defined
- USB 3.0 SuperSpeed – host controller (xHCI) hardware support, no software overhead for out-of-order commands
- USB 2.0 High-speed – Significantly enables command queuing in USB 2.0 drives
- Streams were added to the USB 3.0 SuperSpeed protocol for supporting UAS out-of-order completions
- USB 3 host controller (xHCI) provides hardware support for streams
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- USB Attached SCSI (UAS) Best Practices for Windows 8, page 6
- "USB 2.0 & 3.0 SCSI Compliance test for UAS on EHCI (LOGO)". Msdn.microsoft.com. 2013-07-26. Retrieved 2014-01-20.
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