About Storage Interfaces

Author: Tonya Comer

In today’s data center, customers use Fibre Channel, parallel SCSI, or parallel ATA (PATA) technologies for their hard drive needs. With increasing performance demands, it is more difficult for the current parallel buses to meet these demands.

A logical transition is necessary if SCSI and ATA are to meet the recent performance and management needs; thus the evolution to serial technology. Serial technology will overcome the performance limitations of today’s parallel buses and will enable future growth.

Currently, PATA and parallel SCSI are in the process of transitioning to their respective serial counterparts. Serial ATA (SATA) is the serialized version of ATA, while Serial Attached SCSI (SAS) is the serialized version of SCSI. Labeled the “serial tsunami,” serial technology extends beyond the capabilities used today by providing fundamental storage benefits to mainstream customers and future growth.

The primary difference between serial and parallel architectures is the clock. In serial technology, the clock is embedded in the data. This difference results in data constantly moving through the system from one point to another using the bandwidth to the fullest extent. A parallel architecture clock moves data at a measured pace, resulting in intermittent periods of inactivity. Data using parallel paths simply can’t move as fast. SAS will change the landscape of the disk interconnects because its inherent compatibility with SATA drives will enable unprecedented levels of flexibility with lower overall system cost.

Serial technology addresses the ongoing customer requirements of performance and manageability while meeting increasing customer demands for scalability, density, and data availability at a low cost. SATA technology will expand on PATA’s cost-optimized benefit for non-mission critical applications. SAS extends Ultra320 SCSI to continue to address the performance and reliability needs of enterprise environments.

The latest SCSI interface, Ultra320 SCSI, has an end of life expectation of 2007+, while PATA has reached its maximum limits in throughput. SATA is a replacement for PATA with the same reliability as PATA but not as robust as SCSI technology.

Serial technology is prevalent today and is stable and mature with room to grow. SATA technology has become a viable technology in storage today and will continue to become a viable technology for servers throughout 2004. SAS will announce at 3GB/s, with a future growth path that includes SAS 6GB/s in 2007 and SAS 12GB/s by 2009.

Currently, all parallel SCSI devices must share a common bus of up to 320 MB/s, and all parallel ATA devices must share a common bus of up to 100 MB/s. Serial technology, on the other hand, provides a robust data flow capability; each device on the bus has a dedicated bandwidth of up to 300MB/s.

Serial technology can offer customers more advantages than parallel technologies. These advantages include:

  • Ease of deployment
  • Configuration flexibility and simplicity
  • Point-to-point interface, increasing system reliability and scalability
  • Topologies, scaling with the needs of end users
  • Commonality of components
  • Simplified IT support
  • Improved IOPS
  • More flexible and thinner cable
  • Support for small form factor hard disk drives

Transitioning current PATA and parallel SCSI to their respective serial counterparts, SATA and SAS, enables serial technology to extend beyond the hard drive capabilities used today by providing fundamental storage benefits to mainstream customers and future growth.

By combining industry standards in a universal architecture, SAS and SATA allow customers unprecedented flexibility in choosing the right class of storage to meet their requirements. Both technologies have similar electrical interfaces, giving users the choice of deploying cost-effective SATA drives for low-cost bulk storage or higher performance SCSI drives for mission-critical applications. Customers will benefit from the freedom to choose either SAS or SATA drives or both, depending on storage applications, since they are both operable on the same backplane. Serial technology is the logical evolution that preserves P-SCSI and PATA advantages.

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