Author: Marty Czekalski
When introducing a new interface into a system, several questions generally come to mind such as; what characteristics does the new device possess, what are its specifications and how will it fit into the overall system design?
This development overview article will look at devices that can be used in Serial Attached SCSI (SAS) systems. We will also look at some of these devices’ characteristics and how various standards organizations and committees manage their specifications. Finally, we will talk about where a designer or technical professional can go to get additional information on various components. (For the purposes of this article we will focus primarily on SAS disk drives and will briefly discuss tape drives and Serial ATA (SATA) disk drives.)
In general there is no single specification or standard that covers a disk drive. There are several different standards and specification development organizations that play a part in the overall specification for a drive.
SAS disk drives are expected to be available in both today’s 10K RPM and 15K RPM 3.5″ form factor and emerging 2.5″ server form factor drives. These drives will offer an interface speed of 3Gb/sec., dual ports and full duplex operation. SAS disk drives will typically be built on the same mechanical platforms as parallel SCSI or Fibre Channel (FC) disk drives.
Using the same mechanical platform means the mechanics, platters, heads, actuator, motor, servo and the majority of electronics will be the same in all three types of drives (SAS, parallel SCSI and FC). The SCSI command set used in all three is also the same. Because of this commonality, SAS disk drives will nominally consume the same power as their SCSI cousins. The only difference will be in the physical layer, phy layer, link and transport layer protocols of the interface.
Disk drive physical form factor and connector specifications are documented by the SFF (formally Small Form Factor) Committee (www.sffcommittee.com). By following the links on this web page to the FTP site, you will find the specifications for different form factor drives, connectors and interface locations. The SFF specifications of particular interest for SAS are:
- SFF-8221 Pre-Aligned 2.5″ Drive >10mm Form Factor
- SFF-8223 2.5″ Drive w/Serial Attachment Connector
- SFF-8301 3.5″ Drive Form Factor Dimensions
- SFF-8323 3.5″ Drive w/Serial Attachment Connector
- SFF-8482 Internal Serial Attachment Connector
- SFF-8501 5 ¼” Drive Form Factor Dimensions
- SFF-8523 5 ¼” Drive w/Serial Attachment Connector
The SAS physical layer, phy layer, link and transport layer protocols are part of the SCSI family of standards. This set of standards is produced and maintained by the T10 Committee which is part of the InterNational Committee for Information Technology Standards (INCITS) (www.incits.org). The current SAS specification, ANSI/INCITS 376-2003, can be purchased online at (www.techstreet.com/ncitsgate.html). A working draft of the next revision is available on the T10 website (www.T10.org). Also on this web site are working drafts of the entire family of SCSI specifications that cover all the other aspects of the SCSI architecture, including command sets.
Serial ATA (SATA) disk drives are currently shipping and are typically based on the same mechanical platforms that parallel ATA drives utilize. These drives currently offer an interface speed of 1.5Gb/sec., a single port and half duplex transfers. It is expected that the interface speed of SATA disk drives will increase to 3Gb/sec. within the next year. SATA disk drives have the same basic form factor dimensions as SAS drives and the same SFF Committee specifications apply in both cases.
The SATA physical layer, phy layer, link and transport layer protocol specifications were developed by the SATA and SATA II Working Groups. These specifications can be found on the www.serialata.org web site. Of particular interest for use in SAS systems is the Port Selector, which allows a SATA drive to be connected to two SAS domains (similar to dual port SAS drives) and allows a static fail-over access path to a SATA drive. The SATA 1.0 specification has been turned over to the INCITS T13 Committee and is being incorporated into the current draft of the ATA/ATAPI-7 specification (available on www.T13.org).
Tape drives are usually incorporated into a library or autoloader to facilitate automated backup and archival. These subsystems are typically more bounded in configuration than a disk subsystem and as a result, tape drives have typically been slower to adopt new interfaces than disk drives. For example, many Fibre Channel-connected libraries still use parallel SCSI tape drives connected through a FC-SCSI bridge. It is expected that for connection to SAS infrastructures, tape drives will initially use an approach similar to a bridge. Native SAS-connected tape drives are expected to follow. The latest draft of the SAS specification (SAS 1.1) includes enhancements specifically to allow more efficient operation of tape drives (transport layer retries). The recent introduction of disk-to-disk-to-tape back-up strategies and Information Lifecycle Management (ILM), coupled with the ability of a SAS infrastructure to accept both SAS and SATA disk drives, has created considerable interest in having tape drives included within SAS subsystems to provide highly cost- and performance-effective solutions.
For additional overviews of SAS and SAS tutorials, the SCSI Trade Association website (www.scsita.org) has a broad range of technical information as well as links to member sites.