Introduction of SCSI, iSCSI, and SAS in Linux and which is better.

In this article, we will look at the differences between SCSI, iSCSI, and SAS and will understand in brief how they are co-related.

1. SCSI or Small Computer System Interface
SCSI defines set of standards for connecting and transferring data between computers and different peripheral devices. The most common peripheral devices used to connect via SCSI includes hard disk drives, tape drives, and some scanners/printers.

2. Interfaces of SCSI
Parallel SCSI (SCSI Parallel Interface or SPI)
Serial Attached SCSI (SAS) :- From 2005, SPI was gradually replaced by SAS due to its simpler serial design with fast speed following the same SCSI architecture to function.

3. iSCSI (Internet Small Computer Systems Interface)
iSCSI is an advanced variation of SCSI which provides ability to carry SCSI commands over TCP/IP network.

iSCSI provides block-level access to various peripheral devices, most commonly storage devices by carrying SCSI commands over a TCP/IP network.

iSCSI client is known as an initiator.
And the storage device or the peripheral device on which SCSI command is fired, known as target.

For example:- When i access a remote storage device using iSCSI than my local system is an initiator and the remote storage device is target.

4. SAS :- Serial Attached Storage
SAS is a point-to-point serial protocol which moves data from one point(a system) to another point(various peripheral devices most commonly hard drives and tape drives).

SAS using SCSI command set provides compatibility with SCSI, as well as SATA(Serial ATA), versions 2 or above.

We can make connection of SATA drives to most SAS backplanes or controllers.
However vice versa, connecting SAS drives to SATA backplanes or controllers, is not possible

SCSI, SATA and SAS: Ways of connecting Peripheral devices
Above we have seen these are various ways of connecting(& transferring data) the storage devices, or other peripheral devices to computer system.
However there are some differences of speed and compatibility where lies the difference between them.

In layman terms, we can say that they are connectors that connect the system motherboard usually with the hard drives(and some other peripheral devices too).

5. Which is better SATA, SCSI, or SAS ?

1. SAS is newest in these, 
and hence provides maximum compatibility with SAS, SCSI and SATA drives.

2. The most common format for SATA drives is 7.2K, whereas SAS has two types: 10K and 15K. 
Here ‘K’ refers to the speed at which data platters rotate and writes to the disk.
Hence SAS drives are faster and more reliable than SATA drives.

3. Serial interfaces is an improvement over older parallel SCSI storage environments 
and hence provides better performance, scalability, and also more reliability 
as compared to the parallel interfaces which has data transfer speed limit. 
Hence SAS is considered before SCSI.

4. From the above two points it is clear that SAS holds an advantage over SATA and SCSI drives. 
And then comes SCSI in preference, SATA is preferred above ATA, 
however SATA & ATA are quite old technologies now to consider.
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