USB Attached SCSI (UAS) or USB Attached SCSI Protocol (UASP) is a

computer protocol A communication protocol is a system of rules that allows two or more entities of a communications system to transmit information via any kind of variation of a physical quantity. The protocol defines the rules, syntax, semantics and synchroniza ...

used to move data to and from

USB Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply ( interfacing) between computers, peripherals and other computers. A broad ...

storage devices such as

hard drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magneti ...

s (HDDs),

solid-state drive A solid-state drive (SSD) is a solid-state storage device that uses integrated circuit assemblies to store data Persistence (computer science), persistently, typically using flash memory, and functioning as secondary storage in the Computer ...

s (SSDs), and

thumb drive A USB flash drive (also called a thumb drive) is a data storage device that includes flash memory with an integrated USB interface. It is typically removable, rewritable and much smaller than an optical disc. Most weigh less than . Since first ...

s. UAS depends on the USB protocol, and uses the standard SCSI command set. Use of UAS generally provides faster transfers 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 also be used with devices complying with the slower USB 2.0 standard, assuming use of compatible hardware, firmware and drivers.

Overview

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 The SCSI Trade Association, or SCSITA (sometimes STA), is an industry trade group which exists to promote the use of SCSI technology. It was formed in 1996. , sponsor members include HP, Intel, LSI Logic, PMC-Sierra, and Seagate. Requirements ...

(SCSITA) promotes the UAS technology. The

USB mass-storage device class The USB mass storage device class (also known as USB MSC or UMS) is a set of computing communications protocols, specifically a USB Device Class, defined by the USB Implementers Forum that makes a USB device accessible to a host computing devic ...

(MSC) Working Group develops and maintains the UASP specification; the

USB Implementers Forum The USB Implementers Forum (USB-IF) is a nonprofit organization created to promote and support USB (Universal Serial Bus). Its main activities are the promotion and marketing of USB, Wireless USB, USB On-The-Go, and the maintenance of the sp ...

, 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 USB 3.0, released in November 2008, is the third major version of the Universal Serial Bus (USB) standard for interfacing computers and electronic devices. Among other improvements, USB 3.0 adds the new transfer rate referred to as '' ...

standard, it can also be used at USB 2.0 speeds, assuming compatible hardware. When used with an SSD, UAS is considerably faster than BOT for random reads and writes, but still well below the speed of a native

SATA 3 SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard to ...

interface for certain write tasks. Many

NVMe NVM Express (NVMe) or Non-Volatile Memory Host Controller Interface Specification (NVMHCIS) is an open, logical-device interface specification for accessing a computer's non-volatile storage media usually attached via PCI Express (PCIe) bus. The ...

adapters are USB Attached SCSI devices. The UAS standard (ANSI INCITS 471-2010 and ISO/IEC 14776-251:2014) has been superseded so it should be referred to as UAS-1. A UAS-2 project was started by T10 but cancelled. That effort was resurrected as UAS-3 which is now a published standard (INCITS 572-2021). Apart from being based on later versions of other SCSI standards (e.g. SAM-6 and SPC-6 (both under development)) the technical author described the changes between UAS-1 and UAS-3 as follows: "allow the device to switch data transfers from one command to another before the current command is complete".

Hardware support

A brief hardware roundup in July 2010 by

SemiAccurate ''SemiAccurate'' (S, A in short) is a U.S.-based technology-news and -opinion web site, founded in 2009 by Charlie Demerjian after his departure from ''The Inquirer''. The site lists as its contributors: Charlie Demerjian (the site's founder), T ...

found that

Gigabyte Technology Gigabyte Technology (branded as GIGABYTE or sometimes GIGA-BYTE; formally GIGA-BYTE Technology Co., Ltd.) is a Taiwanese manufacturer and distributor of computer hardware. Gigabyte's principal business is motherboards. It shipped 4.8 million moth ...

had introduced working UAS drivers for their boards using

NEC is a Japanese multinational information technology and electronics corporation, headquartered in Minato, Tokyo. The company was known as the Nippon Electric Company, Limited, before rebranding in 1983 as NEC. It provides IT and network solu ...

Renesas is a Japanese semiconductor manufacturer headquartered in Tokyo, Japan, initially incorporated in 2002 as Renesas Technology, the consolidated entity of the semiconductor units of Hitachi and Mitsubishi excluding their dynamic random-access mem ...

chips, and that—on the hardware level at least—"the LucidPort USB 300 and USB302,

Symwave Microchip Technology Inc. is a publicly-listed American corporation that manufactures microcontroller, mixed-signal, analog and Flash-IP integrated circuits. Its products include microcontrollers ( PIC, dsPIC, AVR and SAM), Serial EEPROM d ...

SW6315,

Texas Instruments Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globall ...

TUSB9260 and the VLI VL700 controllers all support UASP, while as far as we could find, the

ASMedia ASMedia Technology Inc. () is a Taiwanese integrated circuit design company owned by Asus. It produces designs for USB, PCI Express and SATA controllers. Excluding the X570 chipset, all of the AM4 chipsets for AMD's Zen Zen ( zh, t=� ...

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 The Platform Controller Hub (PCH) is a family of Intel's single-chip chipsets, first introduced in 2009. It is the successor to the Intel Hub Architecture, which used two chips - a northbridge and southbridge, and first appeared in the Intel 5 ...

(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 Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washingt ...

added native support for UAS to

Windows 8 Windows 8 is a major release of the Windows NT operating system developed by Microsoft. It was released to manufacturing on August 1, 2012; it was subsequently made available for download via MSDN and TechNet on August 15, 2012, and later to ...

. 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 The Windows Hardware Lab Kit (Windows HLK) is a test automation framework provided by Microsoft to certify devices for Windows. Earlier similarly released frameworks were called ''Windows Hardware Certification Kit'' (Windows HCK) and ''Windows Lo ...

. Apple added native support for UAS to

OS X 10.8 OS X Mountain Lion (version 10.8) is the ninth major release of macOS, Apple Inc.'s desktop and server operating system for Macintosh computers. OS X Mountain Lion was released on July 25, 2012, for purchase and download through Apple's Mac Ap ...

Mountain Lion; drives using UAS show up in (Apple menu -> About This Mac) -> System Information -> Software -> Extensions as IOUSBAttachedSCSI (or IOUSBMassStorageUASDriver, depending on the version of OS X) "Loaded: Yes". Drives listed with "Loaded: No" are defaulting to the older, slower Bulk Only Transport (BOT) mode. This may occur if the drive's USB controller, the Mac's USB port, or any attached USB hub doesn't support UASP mode. The

Linux kernel The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU ope ...

has supported UAS since 8 June 2014 when the version 3.15 was released. However, some distributions of

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which in ...

such as

Ubuntu Ubuntu ( ) is a Linux distribution based on Debian and composed mostly of free and open-source software. Ubuntu is officially released in three editions: '' Desktop'', ''Server'', and ''Core'' for Internet of things devices and robots. All the ...

(from v11.xx onwards) have suffered from issues with the implementation of the UAS protocol. In some non-UAS supported USB HDD drives, the drive is not mountable by the operating system. A reported workaround is to blacklist the UAS module in

modprobe modprobe is a Linux program originally written by Rusty Russell and used to add a loadable kernel module to the Linux kernel or to remove a loadable kernel module from the kernel. It is commonly used indirectly: udev relies upon modprobe to load d ...

FreeBSD FreeBSD is a free and open-source Unix-like operating system descended from the Berkeley Software Distribution (BSD), which was based on Research Unix. The first version of FreeBSD was released in 1993. In 2005, FreeBSD was the most popular op ...

does not support UAS as of August 2018.

Goals

* 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 ** Enables TRIM (UNMAP in SCSI terminology) operation for SSDsNew API allows apps to send "TRIM and Unmap" hints to storage media
* 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 – 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 3.0 host controller (xHCI) provides hardware support for streams

References

External links

USB Attached SCSI Protocol (UASP) v1.0 and Adopters Agreement
2009-06-24 * {{URL, https://web.archive.org/web/20190318084825/https://usb.org/document-library/usb-mass-storage-class-specification-uasp-bootability-v10-and-adopters-agreement, USB Mass Storage Class Specification for UASP Bootability v1.0 and Adopters Agreement, 2013-03-04

(data on t10.org)
USB Attached SCSI Protocol (UASP)
(PDF) USB SCSI Computer storage buses