IDE vs SCSI vs iSCSI

A lot of us are having this question in mind.

Let us clarify that here:

IDE:

The popularity of SCSI is increasing rapidly, but I believe this is due to a misunderstanding. It is often thought that SCSI automatically blows IDE away when it comes to performance. While SCSI does offer a faster throughput, one's activities on the machine affect just how much this performance will really matter. Several factors must be considered when determining which is better for you.

Performance

Most PC's use IDE drives because they are cheap and they perform well. But, to look at performance, you need to look at the entire drive.

Many manufacturer release identical model drives in both IDE and SCSI formats. If you look at these drives, they are identical except for the logic board. this means that the HDA and other drive mechanics are the same. The difference lies in the logic board. The IDE logic board has the disk controller and the built on AT bus interface. The logic board on the SCSI drive contains one extra SBIC chip. Basically, this chip is a SCSI adapter to allow the drive to operate on a SCSI bus. So, structurally, IDE and SCSI drives are the same.

The performance overhead of SCSI over IDE comes from structure of the bus, not the drive. The nature of the SCSI bus allows it much better performance when doing data hungry tasks such as multi-tasking. The SCSI bus controller is capable of controlling the drives without any work by the processor. Also, all drives on a SCSI chain are capable of operating at the same time. With IDE, one is limited to two drives in a chain, and these drives cannot work at the same time. In essence, they must "take turns".

Comparison

In some computers, SCSI is better. As mentioned above, SCSI is a smarter bus than IDE. There are many steps in the SCSI data transfer. But, on OSes that allow multitasking, or if you often use several programs at once, the SCSI drive is a better choice because this extra intelligence of the SCSI bus is used.

SCSI devices can communicate independently from the CPU over the SCSI bus. This is due to the fact that each device has its own embedded controller. Data can then be transferred at high-speeds between the devices without taking any CPU power. IDE, likewise, uses controllers on each device, but they cannot operate at the same time and they do not support command queuing.

Last Thoughts

Finally, let me say that for most people, IDE is just fine and offers very good performance. The reason I believe one does not need to get SCSI, though, is that most users do not use their system in a way that would actually justify the SCSI bus. While the nature of the bus is faster, it takes certain situations to actually need it. Couple this with the significantly higher price, one can see that they can easily live with IDE.

SCSI vs iSCSI:

Let us assume you are asking about parallel or serial attached SCSI vs. iSCSI. iSCSI is the SCSI protocol mapped to TCP/IP and run over standard Ethernet technologies. This allows Ethernet networks to be deployed as SANs at a much lower TCO than Fibre Channel (FC).

Parallel SCSI and serial attached SCSI (SAS) are technologies designed to be inside a box such as DAS or within a storage array. They are not viable SAN technologies at this time.

The iSCSI on Ethernet (10/100/1000/10000) is a good viable external interconnect between application server initiators and storage targets. Parallel SCSI and SAS are good internal interconnects between the server and its internal storage or between the array controller and its drawers of hard disk drives (HDDs).