The difference between hard disk SCSI interface and SAS interface

SAS interface technology can be backward-compatible with SATA. Specifically, the compatibility of the two is mainly embodied in the physical layer and the protocol layer compatibility. In the physical layer, the SAS interface and SATA interface are fully compatible, SATA hard disk can be directly used in the SAS environment, from the interface standard, SATA is a child standard SAS, so the SAS controller can directly control the SATA hard drive, but SAS is not directly used in the SATA environment, Because the SATA controller does not control the SAS hard drive, at the protocol layer, SAS consists of 3 types of protocols for data transmission based on the different devices connected. Where the serial SCSI Protocol (SSP) is used to transmit SCSI commands; SCSI Management Protocol (SMP) for the maintenance and management of connection devices; The SATA Channel Protocol (STP) is used for data transmission between SAS and SATA. So with these 3 protocols, SAS can be seamlessly integrated with SATA and some SCSI devices.

The back panel (backplane) of the SAS system can connect both a dual-port, high-performance SAS Drive and a high-capacity, Low-cost SATA Drive. So SAS drives and SATA drives can exist in one storage system at the same time. However, it should be noted that the SATA system is not compatible with SAS, so SAS drives cannot be connected to the SATA backplane. Because of the compatibility of SAS system, enabling users to use different interface of hard disk to meet the requirements of various applications in capacity or efficiency, so in the expansion of the storage system with more flexibility, so that storage equipment to maximize investment benefits.

In the system, each SAS port can connect up to 16,256 external devices, and SAS take direct point-to-point serial transmission, the rate of transmission up to 3Gbps, it is estimated that there will be 6Gbps or even 12Gbps of high-speed interface appears. The SAS interface has also been greatly improved, providing a 3.5-inch and 2.5-inch interface that can be tailored to the needs of different server environments. SAS relies on the SAS extender to connect more devices to the current extender with 12 ports, but according to the board manufacturer's product development plan, a 28, 36-Port Extender will be introduced in the future to connect SAS devices, host devices, or other SAS extenders.

Compared with the traditional parallel SCSI interface, SAS is not only significantly increased in interface speed (now the mainstream ultra-320mb/sec SCSI speed is the same, and the SAS just start to reach 300mb/sec, the future will reach 600mb/sec or more), And because of the use of serial cable, not only can achieve longer connection distance, but also to improve anti-jamming capability, and this thin cable can also significantly improve the internal cooling of the chassis.

　　The current deficiencies of SAS are mainly as follows:

1 hard disk, control chip types: Only Seagate, the extension and Fujitsu, such as a few hard disk manufacturers launched the SAS interface hard disk, the variety is too small, other manufacturers of SAS hard drive most in the product internal testing phase. In addition, the surrounding SAS controller chip or some of the SAS card type is not much, mostly concentrated in the LSI and Adaptec company hands.

2 Hard drive prices are too expensive: SAS drives are much more expensive than the same volume ultra-hardware SCSI hard drives. The high price has a direct impact on the number of users and the number of channels of digestion, and can not form a mass production of SAS hard drives, the cost of the pressure will in turn lead to price can not be reduced. If users want to make a simple raid level, they need not only buy multiple SAS drives, but also buy expensive raid cards that are basically the same price as the hard drive.

3 The actual transmission speed change is not large: the interface speed of the SAS hard drive does not represent the speed of data transmission, the mechanical structure of the hard disk is limited, now the mechanical structure of SAS hard disk and SCSI hard drive is almost the same. At present, the bottleneck of data transmission is focused on the internal data transmission speed of hard disk, which is determined by the internal mechanism of hard disk and hard disk storage technology, and the speed of disk, which is about 80MBSEC, the performance of SAS hard disk is not improved.

4 User pursuit of mature, stable products: From now launched products, SAS hard disk is more used in high-end 4-way server, and more than 4 server users are not blindly pursuit of high speed hard disk interface technology, the most attractive to them should be mature, stable hardware products, Although the SAS interface server and SCSI interface products in the same speed, stability, but the current technology and products are not mature enough.

However, as Intel and other motherboard chipset manufacturers, Seagate and other hard disk manufacturers and a large number of server manufacturers to promote, SAS related product technology will gradually mature, the price will gradually fall, sooner or later, will become the mainstream server hard disk interface.