Is SAS an advantage

SAS - Serial Attached SCSI


Serial Attached SCSI, SAS for short, is a mass storage interface that transports SCSI commands via a serial interface that is similar to SATA. In the first generation, SAS supports a gross data rate of 3 GBit / s. With SAS 2, the data rate was doubled to 6 GBit / s. With SAS 3 or SAS 12G, 12 GBit / s are provided.

Note: Serial Attached SCSI (SAS) connection technology, which is widespread in servers and storage systems, is rarely found. It has been replaced by PCIe and NVMe.

Overview: SAS - Serial Attached SCSI

versiondescriptionTransfer rate
SAS 1SAS3 GBit / s300 MB / s
SAS 2SAS 6G6 GBit / s600 MB / s
SAS 3.0SAS 12G12 GBit / s1,200 MB / s
SAS 4.0 24 GBit / s3 GB / s

How does SAS work?

A pure point-to-point connection between the host adapter and drive eliminates the need for configuration and simplifies cabling. All SAS components have their own fixed address. The Serial SCSI Protocol (SSP) is responsible for implementing the existing SCSI protocol. It ensures communication between the SAS controller and the SAS device.

SATA + SCSI = SAS

In order to be able to further increase the interface speed of mass storage devices (hard disks), a switch has been made from parallel to serial interfaces.

  • From ATA to SATA.
  • From Centronics / Parallel to USB.

SCSI also had to follow this trend in order to keep up with SATA. It is true that iSCSI and Fiber Chanel are competitors. But these two mass storage interfaces are more suitable for bridging longer distances between the server and RAID arrays. That is why they started developing a serial interface for SCSI.
In principle, SAS works like SATA, which has been expanded to include SCSI commands and greater reliability.

SAS hard drives have many advantages over SATA drives. SAS supports Native Command Queuing (NCQ) with up to 256 commands. By cleverly rearranging the commands, the read head movements and thus the latency times are minimized. This is also available with SATA, but only for 32 commands.
SAS uses the same connectors as SATA. The SAS drives are hot-pluggable right from the start. In addition, the signal level with SATA is ± 250 mV. With SAS they are ± 1.2 V. This is less susceptible to interference and longer cables can be used for SAS.

Dual porting

SAS drives can be connected to multiple hosts. In addition, almost every SAS hard drive has a second port for dual porting.

Parallel operation of SAS and SATA drives

SAS and SATA drives can be operated in parallel on a SAS host adapter. However, this does not work on a SATA host adapter. Only the SAS standard provides a protocol extension with which SAS and SATA devices can be operated in a mixed storage system. For example, SAS with its high speed (high disk speed) for applications and SATA disks as inexpensive storage for backups.
The SAS host adapter can differentiate between SAS and SATA devices and addresses SATA devices with the SATA Tunneling Protocol (STP). The STP packs SATA commands in its own data packets.

Network structure for larger storage systems

Like SATA, SAS is basically a point-to-point interface. However, with the SCSI Management Protocol (SMP), several drives can be interconnected on a SAS interface with several point-to-point connections. An expander, an electronic switch, divides a SAS channel over several ports. Like a switch in a network, the expander coordinates the data traffic and switches the point-to-point connections to the destination port. Each expander can manage up to 128 SAS addresses. Up to 16,384 (128 x 128) devices can be managed in a SAS system.

Now you can understand why SAS 12G with 12 GBit / s makes sense? This is because individual hard drives do not need this data rate. But if you operate several of them on one SAS expander, then each individual hard drive benefits from the faster interface.

Overview: Interfaces for hard disks and removable storage drives

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Computer technology primer

The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memory, interfaces, data storage devices, drives and important hardware components.

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Everything you need to know about computer technology.

Computer technology primer

The computer technology primer is a book about the basics of computer technology, processor technology, semiconductor memory, interfaces, data storage devices, drives and important hardware components.

I want that!