What is 6Gb SAS (Serial-attached SCSI)?
SAS is a new serial interface standard for SCSI (Small Computer System Interface), introduced in 2005 to improve upon the performance of the existing parallel SCSI interface that first appeared in the mid 1980s. As an open standard, SCSI has enjoyed accelerated development and has become the single, most successful storage industry interface used on most servers and in many storage applications, such as Direct Attach Storage, Network Attached Storage and Storage Attached Networks. The SCSI protocol is also used in Fibre Channel, iSCSI and InfiniBand technologies.
The main advantages of SAS over SCSI include:
- Improved performance and reliability, SAS includes the initial 3Gb/s transmission rate and advance command queuing, so operating systems can send multiple read and write requests to hard disks. A point-to-point architecture enabling simultaneous read and write activity over the same port and dual port capability allowing multiple connections, doubling the usable bandwidth and improving fault tolerances
- More flexible, as systems comprising SAS Hard Disk Drives (HDDs) can be connected to SATA (Serial ATA) HDDs
- SAS cabling is more compact providing better airflow, with smaller connectors than parallel SCSI, such as Mini-SAS connectors, plus simplified hot plug connections.
6Gb/s SAS is based on the latest SAS-2 specification and offers a number of significant improvements over 3Gb/s SAS:
- A two-fold increase in performance with a 6Gb per second data transfer rate
- Cleaner systems with Spread Spectrum Clocking (SSC), which varies the clock rate to spread emissions over a wider range of frequencies reducing the peaks of radiated emissions
- Decision Feedback Equalization (DFE) enables greater signal integrity, supporting cable lengths up from 6 metres with 3Gb/s SAS to 10 metres, ensuring storage integrity in large cluster and datacenter deployments
- Expander Self-Discovery and Self-Configuration simplifies the discovery process at power-up, shortening system boot time and reducing interoperability problems
- Standardised zoning, replaces 3 Gb/s SAS vendor proprietary zoning schemes to ease interoperability problems and improve storage efficiency.
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