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For years the parallel interface has been widely used in storage systems. The need for increased bandwidth and flexibility in storage systems made the SCSI and ATA standards an inefficient option.

Common parallel interfaces include SCSI and ATA. The corresponding serial interfaces are SAS and SATA.

SCSI

webopedia.com.

SCSI (pronounced "skuzzy"), the Small Computer System Interface is a set of parallel interface standards developed by the American National Standards Institute (ANSI) for attaching printers, disk drives, scanners and other peripherals to computers.

  1. The first version (SCSI-1), adopted by ANSI in 1986, an 8-bit version, 5 MBps transfer speed, allowed up to eight devices to be connected, a maximum cable length of six meters.
  2. The latest version, 16-bit Ultra-640 (Fast-320) SCSI, introduced in 2003, 640 MBps transfer speed, connecting up to 16 devices, with a 12 meter cable length.

You can attach multiple devices to a single SCSI port, so that SCSI is really an I/O bus rather than simply an interface.

csdn blog. SCSI,其速度、性能和稳定性都比IDE要好,价格当然也要贵得多,主要面向服务器和工作站市场。它由SCSI控制器进行数据操作,SCSI控制器相当于一块小型CPU,有自己的命令集和缓存。

ATA

webopedia.com.

ATA also is called Parallel ATA. Known also as IDE.

Short for Advanced Technology Attachment, a disk drive implementation that integrates the controller on the disk drive itself. There are several versions of ATA, all developed by the Small Form Factor (SFF) Committee.

SAS

webopedia.com.

Short for Serial Attached SCSI, an evolution of parallel SCSI into a point-to-point serial peripheral interface in which controllers are linked directly to disk drives.

SAS is a performance improvement over traditional SCSI because SAS enables multiple devices (up to 128) of different sizes and types to be connected simultaneously with thinner and longer cables; its full-duplex signal transmission supports 3.0Gb/s. In addition, SAS drives can be hot-plugged.

SAS devices can communicate with both SATA and SCSI devices (the backplanes of SAS devices are identical to SATA devices). A key difference between SCSI and SAS devices is the addition in SAS devices of two data ports, each of which resides in a different SAS domain. This enables complete failover redundancy as if one path fails, there is still communication along a separate and independent path.

SATA

webopedia.com.

Often abbreviated SATA or S-ATA, an evolution of the Parallel ATA physical storage interface.

Serial ATA is a serial link -- a single cable with a minimum of four wires creates a point-to-point connection between devices. Transfer rates for Serial ATA begin at 150MBps.

One of the main design advantages of Serial ATA is that the thinner serial cables facilitate more efficient airflow inside a form factor and also allow for smaller chassis designs. In contrast, IDE cables used in parallel ATA systems are bulkier than Serial ATA cables and can only extend to 40cm long, while Serial ATA cables can extend up to one meter.

The Benefits of SAS & SATA

webopedia.com.

Serial interfaces offer an improvement over older parallel SCSI (with a serial version) in storage applications and environments. These benefits include better performance, better scalability, and also better reliability as the parallel interfaces are at their limits of speed with reliable data transfers.

SAS and SATA drives can also operate in the same environment while SCSI and ATA cannot. For example, using faster SAS drives for primary storage and offloading older data to cheaper SATA disks in the same subsystem, something that could not be achieved with SCSI and ATA.

Why is SATA faster than PATA?

A long time ago, parallel was preferred because the limitations to speed were in the copper wires themselves. You simply couldn't push a very fast signal through, so the need to put multiple signals together was used. However, as you know, technology changes. Wiring/routing has advanced significantly to the point where the individual wire delay can be minimal (very fast). The main problem now is skew, crosstalk, noise, etc. These problems weren't very big when we were running at 1 MHz signals but they're a huge problem at the GHz range over a long distance (across the motherboard). So, as conditions change, methods must change. So the solution was obvious, use less wires. Instead of having 32 wires running at 100MHz - 133MHz, we can have 2 wires running at 2.5 GHz - 6 GHz. And the ability to push serial link speeds isn't difficult whereas the ability to push parallel link speeds increases exponentially. So, for right now, serial seems to be outpacing parallel.

Another advantage is granularity. With a parallel link, you need to organize your data and send them in chunks from one point to another. Parallel links are expensive (many wires) so you can't have many of these, so you'd need controllers to share these wires which leads to all sorts of scheduling and resource conflicts, etc. With serial, the links are so cheap that there's no need to share them. Each device gets their own link. For areas where large bandwidth is needed, simply put many serial links together. They don't interfere with each other as they're independent. Synchronization is done at the receiving end as opposed to having to be timed in the wires themselves. It does require a more sophisticated receiving end than plain parallel, but the transfer rates you can get are much greater.

So in short, serial is more flexible, cheaper and ultimately easier to improve upon in the future.AnAndTechForum.

Some things like PCI Express do the best of both worlds, they do a parallel set of serial connections (the 16x port on your motherboard has 16 serial connections). By doing that each line does not need to be in perfect sync with the other lines, just as long as the controller at the other end can reorder the “packets” of data as they come in using the correct order.

It is easier to make a single connection go 16 times faster than 8 connections go 2 times faster once you get to very high frequencies.
HowToGeek: why is serial data transmission fater than parallel.