SCSI / ATA Translation
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SCSI / ATA Translation (SAT) is a set of standards developed by the T10 subcommittee, defining how to communicate with ATA devices through a SCSI application layer. The standard attempts to be consistent with the SCSI architectural model, the SCSI Primary Commands, and the SCSI Block Commands standards.
The standard allows for translation of common SCSI Block Commands such as:
The standard also provides the ability to control exactly what ATA operations are executed on a target device by defining two new SCSI operation codes:
- ATA PASS THROUGH (Ax, 12-byte) – 28-bit ATA command without
- ATA PASS THROUGH (8x, 16-byte) – 28- or 48-bit ATA command without
The first SAT standard was finalized in 2007 and published as ANSI INCITS 431–2007. It was succeeded by SAT-2 published as INCITS 465 in 2009, and SAT-3, which was finalized by T10 and is expected to be published as INCITS 517 in 2014. SAT-4 is in development. SAT has also been adopted in 2008 as an ISO/IEC JTC 1/SC 25 standard, namely ISO/IEC 14776-921.
SAT-2 was finalized in 2009. Significant additions in SAT-2 are ATAPI translations, NCQ control, persistent reservations, non-volatile cache translation, and ATA security mode translations. The standard also defines a new data structure returned in the sense data known as the ATA Return Descriptor which contains the ATA taskfile registers. SAT-2 was promulgated as ISO/IEC 14776–922 in 2011.
SAT-3 was finalized in 2014, and SAT-4 in 2016. Since the standards have become ANSI standards, the drafts are inaccessible to the public. SAT-4 added a 32-byte ATA PASS-THROUGH command. This version of the command support additional
ICC fields used by some ATA commands.
Work on SAT-5 began in 2017. As of July 2020[update], it has not yet become a standard, making its drafts freely available.
SAT is useful for enabling ATA-device-specific commands in a number of scenarios:
- SATA disks attached to SAS controllers
- [P]ATA or SATA disks attached via USB bridges (which actually speak SCSI over the wire either using the older USB Mass Storage Bulk-Only Transfer protocol or the newer USB Attached SCSI protocol).
- [P]ATA or SATA disks attached via FireWire bridges (which speak SCSI SBP-2 over the wire)
A problem with many SAT implementations is that when two or more ATA devices are attached to the same SAT interface, the ATA passthrough only serves one device. This causes issues with fetching S.M.A.R.T. data, since only one device can be addressed.
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- Douglas Gilbert, "Changes to Storage Standards", Linux File System + IO Workshop, San Jose, February 2007
- "IEC - ISO/IEC JTC 1/SC 25 Dashboard > Projects: Work programme, Publications, Maintenance cycle, Project files, TC/SC in figures". Iec.ch. 2011-05-31. Retrieved 2014-01-21.
- "libata: Support for an ATA PASS-THROUGH(32) command". Linux kernel Patchwork. Jun 24, 2017.
- "T10 Working Drafts: SCSI / ATA Translation". www.t10.org. Retrieved 1 July 2020.
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With more transports "hiding" SATA disks (and other S-ATAPI devices) behind a SCSI command set, accessing special features like SMART information becomes a challenge. [...] or somewhere on the interconnect between the host computer and the SATA devices (e.g. a RAID made of SATA disks and the RAID talks "SCSI" to the host computer). Note that in the latter case, this code does not solve the addressing issue (i.e. which SATA disk to address behind the logical SCSI (RAID) interface).