By Daniel D. Reno, Director, Technical Marketing, Western Digital and Member-at-Large, STA Board of Director
Enterprise customers demand the highest levels of quality and reliability for the systems they deploy. To deliver these robust solutions, industry leaders rely on the most advanced technologies available as a foundation of their product solutions. Within these products, the SAS (Serial Attached SCSI) interface has become a critical component for building the ‘Unstoppable Datacenter’.
But what are the key attributes provided within the SAS infrastructure? As recently demonstrated in the heart of Silicon Valley at the SCSI Trade Association’s (STA) May 2012 Technology Showcase, SAS offers multiple levels of protection that provide uncompromising data integrity and robust features. These SAS protection highlights include:
- “Built-in” resiliency through multiple layers of the protocol
- System-level protection with T10 Protection Information (PI), to defend against transmission, translation, and routing errors for data in-flight
- Failover features which enable Advanced SAS RAID capabilities to maximize storage availability by keeping your enterprise systems running even during single points of failure
The SAS architecture delivers protection at every layer of the protocol. As depicted in Figure 1, SAS has protection at each of the Physical, Link, and Transport layers. The protection mechanism assures that the electrical signals are optimized, the connection is setup and validated, and that the payload is checked and double checked for accuracy.
Figure 1: SAS protection at the Link, PHY and Physical layer of the protocol
Figure 2: Signal to noise ratios before and after Decision Feedback Equalization (DFE)
System-level Protection Information
Historically, a number of system developers have provided proprietary systems with additional protection application protection. The T10-PI is an industry standard implementation for providing additional protection to the typical user data. As shown in Figure 3, a typical user data block consists of 512 bytes:
With the addition of T10-PI, eight additional bytes of protection information are appended throughout the entire data transfers from the Initiator (e.g. the Host Bus Adapter) to the Target (e.g. a Storage device such an HDD). When transmitted to the Target, these bytes are saved internally with the user data to be both checked and transferred back to the Initiator when the user information is retrieved. There are 3 separate components to the 8 bytes as show in Figure 4:
- Guard Tag: 2 bytes of CRC (Cyclical Redundancy Code) information to validate the data was transmitted correctly
- App Tag: 2 bytes of System level information to define a unique application from where the data emanated
- Reference Tag: 4 bytes of Logical Block Address information for the command
Figure 4: T10-PI adds eight bytes of protection to user data block
Advanced RAID Failover capabilities
SAS provides two separate Ports into the Target device; example is the HDDs as shown in Figure 5. Attached to these ports can be two entirely separate initiators as depicted by two RAID BLADE servers. Based on the communication of this Advanced RAID application within this system, these separate paths can assure that the data to the targets is still accessible if any of the paths are ever broken. This built in redundancy assures that user data can always be accessed!
Figure 5: SAS enables two RAID BLADE servers for built-in redundancy for HDDs
The Unstoppable Datacenter
SAS-based solutions deliver confidence and peace of mind to your customers, protecting data and making it readily accessible. As an industry led interface, you can find additional information about SAS at the WWW.SCSITA.ORG site, as well as a list of many of the companies whom support developing these capabilities to ensure your systems run with absolute trust… to provide you with YOUR Unstoppable Datacenter!