This article is for informational purposes only and no longer being updated by Seagate.

Serial Attached SCSI - Frequently Asked Questions

Heard the buzz about Serial Attached SCSI and want to learn more? Below are answers to frequently asked questions about this breakthrough in enterprise storage technology.

1. What is the difference between Serial Attached SCSI and Parallel SCSI?

Serial Attached SCSI (SAS) is an evolutionary development of parallel SCSI, a proven technology which has been the foundation of enterprise storage for over two decades. SAS leverages the rock-solid reliability of parallel SCSI while offering dramatic improvements in performance, scalability and compatibility.

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2. Are parallel interfaces faster than serial interfaces?

In the past, parallel interfaces were preferable because their multiple data paths enabled greater throughput than the single data path of serial interfaces. However, recent advancements in VLSI technology have enabled serial transfer rates to make a huge leap forward, without the complex timing issues that hinder further parallel interface development. Compared to parallel SCSI, Serial Attached SCSI features higher throughput now and greater potential for advancement in the future.

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3. Does this mean parallel SCSI is now obsolete?

Not necessarily. Parallel SCSI has long played a key role in enterprise data storage, and will continue to have a presence. That said, Serial Attached SCSI is a compelling complement that matches the superb reliability and robustness of its acclaimed predecessor, while significantly expanding the SCSI envelope in terms of speed, scalability and flexibility.

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4. Will the migration path from parallel SCSI to SAS be difficult?

Ease of migration was a primary consideration when Seagate and other industry figures collaborated to define Serial Attached SCSI standards. SAS was specifically engineered to leverage existing SCSI command sets, preserving your investment in storage management and enterprise application software. And unlike ATA solutions, no system-level workarounds are required to deploy SAS drives.

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5. What is the difference between Serial Attached SCSI and Serial ATA?

Serial Attached SCSI is an enterprise-class solution that goes beyond its interface to deliver the superior performance, reliability and scalability demanded in mission-critical applications. Serial ATA is primarily a desktop-class solution suitable for use in light-duty environments where low cost is the highest priority.

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6. Are SAS and Serial ATA compatible?

Yes, compatibility with Serial ATA is indeed a core feature of Serial Attached SCSI. SAS backplanes and Host Bus Adapters (HBA) are fully compatible with Serial ATA, enabling connectivity for both types of drives on a common backplane, lowering infrastructure costs. SAS enclosures offer tremendous flexibility for growing businesses who can deploy Serial ATA drives when storage requirements are modest, then seamlessly add SAS drives as evolving business needs dictate. This guarantees the freedom to specify the optimal drive for the application: SAS for transactional and online performance and reliability, Serial ATA for nearline and backup and restore duties.

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7. What is a Serial Attached SCSI expander?

Expanders are the key to Serial Attached SCSI's remarkable scalability. Each of these low-cost switches enables up to 128 point-to-point connections to be made from a single HBA/enclosure, and a total of 16,384 SAS devices can be aggregated while preserving performance and reliability. By contrast, parallel SCSI imposes a limit of fifteen devices per SCSI chain and severely limits total cable length.

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8. Will Serial Attached SCSI be available in the new 2.5-inch Enterprise form factor?

Absolutely! Serial Attached SCSI's compact cabling and connectors are a perfect match for these smaller devices. Boasting significant space savings, higher energy efficiency and improved airflow, 2.5 inch SAS drives enable consolidation of server/storage hardware into more compact configurations with increased drive density, yielding higher IOPS/U, lower system Total Cost of Ownership (TCO) and improved server CPU cooling.

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9. Does SAS replace Fibre Channel?

Not at all. Serial Attached SCSI and Fibre Channel are both advanced, enterprise-class solutions with specific attributes that influence their suitability for a given application. SAS offers outstanding performance and exceptional scalability on a local level. Fibre Channel is well suited for large enterprise use [e.g., a Storage Area Network (SAN)] due to its support of up to 16 million addresses and maximum cabling distance of ten kilometres.

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10. When will Serial Attach SCSI drives be available?

Serial Attached SCSI is available now. Seagate began ramping up development of Serial Attached SCSI drives in mid 2003, with widespread availability of SAS solutions beginning in early Autumn 2004.

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