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SAS: A Key Fabric in Storage World

In our previous blog on SAS, we discussed about SAS 24G new encoding and features. In the series of SAS blogs, here we shed some light on other generations of SAS that are still hot in the market.

SAS generations

SAS was announced in 2004, supporting data rates till 3Gbps for SAS drives and was compatible with SATA I (1.5Gbps) through an idle interleaving process. Bandwidth scalability in SAS is achieved through connection aggregation. In this model, each SAS connection between common endpoints acts as a logically bonded channel, and bandwidth increases as individual channels are combined. By 2008, SAS-2 devices with 6Gbps data rate gained volumes in the enterprise domain. In 2012, with fourth-generation SAS (SAS-3), interface bandwidth doubled and reached to 12Gbps. SAS-3 addresses signal quality through “transmitter training.” SAS is already mapped to 24Gbps and may go considerably farther in upcoming generations. There are many base features of SAS that are still used. We will discuss some of the base features in this blog.

SAS was developed to address the limitation of the traditional parallel SCSI, I/O and direct-attached storage requirements. SAS provides manageability, performance and reliability, yet being logically compatible to SCSI. Like parallel SCSI, it is a data-transfer model designed to move data to and from computer storage devices such as hard drives. Unlike SCSI, SAS is a point-to-point protocol and allows for much higher speed data transfers which is not possible with parallel SCSI. It uses the standard SCSI command set for communicating with SAS End devices. The SAS protocol was developed and is maintained by the T10 technical committee of the International Committee for Information Technology Standards (INCITS). SAS is not completely new, but it unifies the best of two worlds.

SAS architecture

The first aspect is the serialized transmission of data, which requires only a few physical connections, the cable and connector are smaller in size, allowing vendors to significantly upgrade the storage density of a storage array. Moving from parallel to serialized operations has eliminated the use of a bus as well. The second important factor for SAS is the popular and powerful SCSI protocol. SAS provides enterprise-class robustness, protection of investments in compatible SCSI software and applications, and, since it is compatible with SATA (Serial Advanced Technology Attachment), SAS offers a better choice of direct and attached data storage devices in a single SAS system.

SAS offers the following distinct advantages over parallel SCSI:

  • Up to 65,535 devices (through expanders)
  • Eradicates termination and clock skew issues
  • Both single and dual-port devices
  • Point-to-point technology, not subjected to resource contention issues
  • SAS cards can communicate with both SAS and SATA devices

SAS vs SCSI table

To know more about Synopsys VIP in storage domain – SAS, SATA, and NVMe, please visit http://synopsys.com/vip.

Also read our recent blogs on storage, and keep visiting for upcoming blogs on SAS, SATA, and NVMe.

Authored by Mansi Chadha

 

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