Robust Dual-Port and Multipath for High Availability and Efficient Sharing
by Jeremiah Tussey
Serial-attached SCSI (SAS) is the robust choice for dual-port and multipath solutions, and is critical for high availability and efficiently sharing HDDs and SSDs in an ecosystem. End-to-end data integrity is also an important need for many enterprise applications. Only SAS supports the full SCSI command set, with full duplex I/O and multipath failover connectivity, which enables this form of reliability and availability, ensuring that the data is transferred quickly and error-free.
Reliability and Availability
SAS has been shipping dual-port drives since its inception. A benefit of SAS-based storage subsystems is the native dual-port capability of each SAS drive, providing a redundant path to each drive in the event of a controller-level failover. In addition, multi-initiator capability can be achieved with a SAS Fabric implementation through SAS expanders. This can be further enhanced with simultaneous access and dual-active port support, in addition to the inherent redundant paths to the SAS drives.
An application with a standard multipath failover implementation, as shown in Figure 1, connects multiple SAS controllers to multiple embedded SAS expanders, allowing multipath access to both ports of SAS drives through dual “domains”. If one controller fails, the other controller maintains accessibility to both ports of the SAS drives. If a cable fails or is removed, or if one of the embedded SAS expanders fail, there is still accessibility through the alternate “domain” connection. However, when everything is operating nominally, both controllers have access to both “domains”, allowing multiple connections and commands to access the SAS drives efficiently. Thus, increased bandwidth can be achieved, in addition to the reliable, redundant protection afforded by this topology implementation. Using multipath and dual-port connectivity, users can see performance improvements and eliminate single points of failure collectively. Since most storage failures have a tendency to actually be interconnecting issues, the multipath capability will keep operations running effortlessly.
Innovating for Bandwidth Growth and Increased Availability
The SAS marketplace today is already leveraging the existing 12Gb/s SAS infrastructure for rapid deployment of dual-port products. With the recent addition of MultiLink SAS™ capability to the feature list, further enhancing the SAS value proposition, the SAS roadmap progression shows a clear future with increasing bandwidth and dual-port capabilities to accommodate unique SSD requirements:
- 12Gb/s SAS dual port, two streams (2400MBps)
- 12Gb/s SAS dual port, four streams (4800MBps)
- 12Gb/s MultiLink SAS, four+ streams (4800MBps+)
- 24Gb/s SAS dual port, two streams (4800MBps)
- 24Gb/s MultiLink SAS, four+ streams (9600MBps+)
The reliability, availability and innovation for bandwidth growth examples provided here are what make SAS the robust choice for dual-port and multipath solutions, satisfying user need for storage continuity options in the data center. Please visit scsita.org for the latest SAS roadmaps, features, and capabilities.
Note: MultiLink SAS is a trademark of the SCSI Trade Association.