Difference Between SAS and SCSI

Various storage devices use different technology to transfer data to and from computers or other devices. Serial Attached SCSI (SAS) and SCSI are two of the most common interfaces for connecting peripheral storage devices to a computer. Both the technologies are used for different storage purposes as per capacity, speed, and requirements.  

SAS vs SCSI

The main difference between SAS and SCSI is that SAS is a much-upgraded technology that has evolved from the existing SCSI storage drives. In SAS, the point-to-point link has been introduced for serial communication over the traditional parallel connection of the SCSI storage technology. Both the transfer rate and capacity of SAS are also much higher than SCSI.   

SAS and SCSI

When SAS was introduced in the market, its data transfer rate was 3.0 Gbit/s. When compared to the latest version today, the transfer rate has increased by fifteen folds. A typical SAS system comprises four main components, namely, an initiator, a target, a service delivery subsystem, and expanders.  

SCSI is more appropriate for storage devices like DVD drivers, CD writers, scanners, etc. It is used to deliver data at a faster rate but its capacity is small. The SCSI protocol uses a controller that is responsible for transferring data between various devices and the SCSI bus. 

Comparison Table Between SAS and SCSI

Parameters of ComparisonSASSCSI
Full-FormSAS stands for Serial Attached SCSI.SCSI stands for Small Computer System Interface. 
InventionSAS was introduced in the market in 2004 with much higher speed. The first version of SCSI (SCSI-1) was invented in 1986.
DefinitionSAS is an evolved technology of SCSI devices that transferred from parallel interface to point-to-point links. SCSI is a standard interface that helps in connecting and transferring data between computers or other peripheral devices. 
Data TransferThe latest version of SAS being developed (SAS-5) has a data transfer rate of 45 Gbit/s. Both the external and internal transmission rates are higher and they can reach up to 10000 RPM. 
Device ConnectionSAS can connect up to 128 devices at a single time and it also allows hot plugging. SCSI can connect up to 16 peripheral devices using a single bus and a host adapter. 
CapacitySAS devices have much larger capacities. SCSI has comparatively less capacity than SAS.
Duel OperationSAS supports full-duplex mode operation for maximum data throughputSCSI supports half-duplex mode operation. 
UseThis is an ideal solution for businesses that require storage and archiving huge amounts of data. It is used in enterprise computing with high-end workstations. 

What is SAS?

Hard drives and tape drives can transfer using the serial attached SCSI (SAS) protocol. It is a point-to-point communication technology that is responsible for transferring data to and from various computer storage devices. It is comparatively a new technology than SCSI and has a transfer rate of up to 45 Gbit/s. In SAS, the parallel SCSI was replaced by point-to-point links because the latter one can reduce address conflicts. It also ensures maximum bandwidth for which device is connected by proving them with the individual signal paths.

SAS is incorporated with advanced data transmission mode as it supports full-duplex mode than maximizing the effective data throughput. SAS is also known for connecting several devices (generally up to 128) at the same time but with the help of expanders, it can connect several thousands of devices at once. SAS also has the advantage of hot-plugging which allows the creation of alternative pathways for data transmits. It also eliminates the clock skew issue that exists in the SCSI protocol. 

SAS can be used in compact systems (hard drives) and due to its very large storage capacity, it is used in businesses for data archiving purposes. It also offers compatibility with SATA (Serial ATA) from version 2 and above. It also features dual ports that allow redundant backplanes or multipath I/O. 

What is SCSI?

When SCSI was first launched in the market, in the late 1980s, it was considered a very revolutionary approach towards data transfer. The original SCSI technology used a parallel bus which evolved after almost two decades to serail point-to-point combination and was released as SAS.

The controller that is used in the SCSI system is either attached inside the motherboard or to a host adapter that is attached via the expansion slot in the motherboard. This type of system has a SCSI bus and each device is assigned a specific number in these SCSI buses. 

There are two types of bus, narrow bus, and wider bus. The narrow bus has a number assigned from 0 to 7 whereas the wider bus has a number assigned from 0 to 14. The number assigned to each device is known as SCSI ID. And, the device from or to which data transmission takes place is connected to the CPU through interfaces and buses. 

In SCSI, the average access time is very short and it is also known for very high RPM. SCSI is also a very reliable technology that is used in high-end workstations. It is also known for very low CPU usage and instated of hot plugging, it has the feature of hot-swapping. 

Main Differences Between SAS and SCSI

  1.  SAS is an evolved version of SCSI that was launched in 2004 whereas SCSI was launched in 1986.
  2. SAS is an upgraded technology of SCSI devices that transferred from using the parallel interface to point-to-point links whereas SCSI is a standard interface that helps in connecting and transferring data between computers or other peripheral devices. 
  3. The data transfer rate of SAS is in gigabytes whereas for SCSI it is in megabytes. 
  4. SAS can connect up to 128 devices at a single time and it also allows hot-plugging whereas SCSI can connect only up to 16 devices using a single bus.
  5. SAS devices have much larger capacities as compared to SCSI.
  6. SAS supports full-duplex mode operation for maximum data throughput whereas SCSI uses half-duplex mode to operate. 

Conclusion

 One very common mistake that is often caused by people is that they confuse SAS and SCSI as storage devices. It is important to understand that both SAS and SCSI are different types of interfaces that help in connecting storage devices to the computer. It is up to a specific requirement of the job as to which interface is suitable for various industrial or personal purposes. 

References

  1. https://ieeexplore.ieee.org/abstract/document/1584027/
  2. https://www.intel.in/content/dam/www/programmable/us/en/pdfs/literature/hb/arria-ii-gx/aiigx_52001.pdf
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