Differences between the SCSI interface and the SAS Interface

SAS (Serial Attached SCSI) is a new generation of SCSI technology, which is the same as the popular Serial ATA (SATA) hard drive. It uses Serial technology to achieve higher transmission speeds, and improve the internal space by shortening the connection line. SAS is a new interface developed after the parallel SCSI interface. This interface is designed to improve the efficiency, availability, and scalability of the storage system and provide compatibility with SATA hard disks.

SAS interface technology is backward compatible with SATA. Specifically, the compatibility between the two is mainly reflected in the compatibility between the physical layer and the protocol layer. In the physical layer, the SAS interface is fully compatible with the SATA interface, and the SATA hard disk can be directly used in the SAS environment. In terms of interface standards, SATA is a sub-standard of SAS, therefore, SAS controllers can directly control SATA hard disks, but SAS cannot be directly used in SATA environments, because SATA controllers cannot control SAS hard disks. On the protocol layer, SAS consists of three types of protocols, which are used for data transmission based on different connected devices. Serial SCSI protocol (SSP) is used to transmit SCSI commands; SCSI Management Protocol (SMP) is used to maintain and manage connected devices; SATA Channel Protocol (STP) it is used for data transmission between SAS and SATA. With these three protocols, SAS can be seamlessly integrated with SATA and some SCSI devices.

The Backplane of the SAS system can be connected to a SAS drive with dual-port and high performance, or to a high-capacity and low-cost SATA drive. Therefore, SAS drives and SATA drives can be stored in a storage system at the same time. Note that the SATA system is not compatible with SAS, so the SAS drive cannot be connected to the SATA backboard. Because of the compatibility of the SAS system, users can use hard disks with different interfaces to meet the capacity or performance requirements of various applications. Therefore, they have more flexibility when expanding the storage system, maximize the investment efficiency of storage devices.

In the system, each SAS port can connect up to 16256 external devices, and SAS uses a direct point-to-point serial transmission mode, the transmission speed is up to 3 Gbps, it is estimated that there will be 6 Gbps or even 12 Gbps high-speed interfaces in the future. The SAS interface has also been greatly improved. It also provides interfaces of 3.5 inch and 2.5 inch, so it can meet the needs of different server environments. SAS relies on SAS extenders to connect more devices. Currently, the majority of Extenders are 12 ports. However, according to the product R & D Plan of the board manufacturer, ports 28 and 36 will be introduced in the future, to connect SAS devices, host devices, or other SAS extenders.

Compared with traditional parallel SCSI interfaces, SAS not only significantly improves the interface speed (currently, the mainstream Ultra 320 SCSI speed is 320 MB/sec, SAS has just started to reach 300 MB/sec, and will reach 600 MB/sec or more in the future). In addition, because of the serial cable, it can not only achieve a longer connection distance, it can also improve the anti-interference capability, and this thin cable can also significantly improve the internal heat dissipation of the chassis.

The current shortcomings of SAS mainly include the following:
1) there are few categories of hard disks and control chips: Only a few hard disk manufacturers, such as Seagate, Mattel, and Fujitsu, have launched SAS interface hard disks with few categories, most SAS hard disks of other manufacturers are in the internal testing stage. In addition, there are not many peripheral SAS controller chips or some SAS adapter cards, most of which are concentrated in LSI and Adaptec.
2) hard drive prices are too expensive: SAS hard drive is more expensive than Ultra 320 SCSI hard drive of the same capacity. The continuous high price directly affects the user's purchase quantity and the number of channels to digest, and cannot form a mass production of SAS hard disks, the cost pressure will in turn lead to the price cannot be reduced. If you want to create a simple RAID level, you need not only to purchase multiple SAS hard disks, but also to purchase expensive RAID cards. The price is basically the same as that of hard disks.
3) The actual transmission speed does not change much: The interface speed of the SAS hard disk does not represent the data transmission speed. Due to the limitation of the hard disk mechanical structure, the mechanical structure of the SAS hard disk is almost the same as that of the SCSI hard disk. At present, the bottleneck of data transmission is the internal data transmission speed of the hard disk determined by the internal mechanical mechanism of the hard disk, hard disk storage technology, and disk speed, that is, about 80 mbsec, the performance of SAS hard disks is not significantly improved.
4) users are pursuing mature and stable products: from the current product perspective, SAS hard disks are more applied to high-end 4-way servers, the four or more server users are not blindly pursuing high-speed hard disk interface technology. What attracts them most is mature and stable hardware products, although the speed and stability of SAS and SCSI interfaces are similar, the current technologies and products are not mature enough.

However, as hard drive manufacturers such as Intel and other Motherboard chipset manufacturers, Seagate and many other server manufacturers are pushing forward, SAS-related products and technologies will gradually mature and prices will gradually fall, it will become a mainstream interface for server hard disks sooner or later