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Last updated: Apr 04, 2024

SCSI (Small Computer System Interface) - What is it?

SCSI (Small Computer System Interface) is a smart bus peripheral interface that uses a microprocessor. The SCSI interface is used in many applications, such as hard drive designs and other computer device interface designs. One common thing about the SCSI interface is its devices are generally backward-compatible.

In other words, if you bring an older SCSI peripheral device and attach it to a computer built to support newer SCSI standards. The older peripheral device will still be read by the computer - however, at a slower data read rate.

In modern-day personal computing, SCSI interfaces are becoming obsolete, being mainly replaced with Universal Serial Bus (USB). But, in enterprise computing, SCSI is still being used, especially in virtual server computing.

What is SCSI VMware?

SCSI was initially developed by Shugart Associates and NCR Corporation in 1981; it was called Shugart Associates System Interface (SASI). But in 1986, the American National Standards Institute (ANSI) approved for it to be called SCSI (pronounced “scuzzy”); SCSI was introduced as a modified version of SASI.

SCSI uses a controller to transmit data and power SCSI-enabled devices. It’s a fairly fast data transfer protocol, offering up to 320MBps. Being an old name in the field – over 20 years – SCSI is gradually getting replaced with modernized interfaces, but there are places where it still has high relevance – places like server environments.

One major problem with SCSI protocol has limited system BIOS support, and you have to configure it individually for each computer used. Different SCSI types have different specifications in terms of speed, bus width, and connectors – and this can be quite confusing.

The term “SCSI VMware” may be referred to as the SCSI storage controller used by a vSphere virtual machine. When you create a new VM on VMware vSphere, a storage controller is automatically created in the VM configuration. Also, if you connect a virtual disk or disc to a different storage controller, then you will have two storage controllers for the VM you just created – more can be added if you wish.

SCSI Types and Standards

There are three main variations of SCSI used in different applications. The most recent type was introduced in 1995, which has been quite a long time ago.

  • SCSI-1: This first type was introduced in 1986 and is not obsolete. It has a bus width of 8 bits and delivers 5MHz speed.
  • SCSI-2: Introduced in 1994, this SCSI came with Common Command Set (CCS) and offered twice the clock speed of its predecessor – 10MHz. It allows command queuing so devices can store commands from the host computer. Also, as expected, this version offered a wider bus width of up to 16 bits and supports up to 15 devices.
  • SCSI-3: Introduced in 1995, this version came with a lot of improvements from the older ones, including a series of smaller standards.

However, each of these SCSI types has individual variations too. For example, the SCSI-3 had specifications designated with the term “Ultra.”

  • Ultra - SPI variations
  • Ultra2 - SPI-2 variations
  • Ultra3 - SPI-3 variations

SCSI and Devices

Each SCSI device has a unique identifier (ID). The ID is what signifies the specifications of the SCSI controller. So to say, if a SCSI bus is built to support sixteen devices, the ID will range between 0 and 15.

Internal devices connect to a SCSI controller using a ribbon cable, while external devices connect in a daisy chain – where each device connects to the next one in line – using a thick, round cable. (Note: Serial Attached SCSI devices use SATA cables). External SCSI devices typically have dual connectors and different SCSI variations use different connectors.

While not all devices are compatible with every level of SCSI, SCSI standards are backward-compatible. This means a SCSI device built to support newer SCSI standards can still read an older peripheral device but at a slower speed. Due to the bulky nature of SCSI interfaces, they are rapidly being replaced with Universal Serial Bus (USB) in today’s personal computing devices.

Where is SCSI Used?

SCSI is not the preferred choice in personal computing devices, but it is surely the best interface for servers, workstations, and mainframes. Also, SCSI interfaces are added in computers for scanner connections and connecting other devices like DVD-RAM, CD-Rs, Zip drives, and hard drives.

But, aside from workstation and server applications, SCSI has become less popular in computers generally. Alternative interfaces like USB and FireWire are the latest trends. In network servers, SCSI remains the best as it allows you to pair multiple hard drives into a RAID level with support for “hot-swapping.”

Notwithstanding, you can have a SCSI hard drive in a PC with one or more IDE disk drives; this won’t change the boot order, the IDE drive will remain the boot drive, and the newly installed SCSI drive will serve as additional storage.

To sum it all up, SCSI is commonly used in data centers, enterprise storage systems, and environments where virtual servers are used.

How SCSI Works

A typical computer features different “busses.” Busses are like a “route” that conveys information from one device to another. Universal Serial Bus (USB) is one of the “busses” a computer uses for data transfer and other functions – SCSI is one of the oldest types of busses used by computers.

USB can efficiently carry small data and electricity suitable for “not-so-bulky” data transfers and to power a seemingly small computer device. A USB interface cannot handle much power needed to power a whole computer, server, or multiple drives simultaneously, and that’s where SCSI comes in handy.

Although the first time SCSI was introduced, it was termed a “Small” Computer System Interface, over time, it has literally outgrown the “small” designation to become one of the fastest bus types for connecting multiple devices simultaneously or heavy printers and scanners. SCSI works like every other computer bus, but the speed and functionalities differ.

Common SCSI Components

  • Initiator: This component of the SCSI initiates the request for connection by the SCSI device and also receives responses from the SCSI device. It comes in different forms and can exist within a host bus adapter or integrated into a server’s system board. 
  • Target: Targets are usually physical storage devices (and in some cases, software-based SCSI targets exist). These targets can be hard drives, scanners, printers, a RAID array, or any other storage device – it can also be non-storage hardware.
  • Service Delivery Subsystem: Facilitates communication between the initiator and the target via a cable. 
  • Expander: This component works only with serial-attached SCSI (SAS); it allows for multiple SAS devices to use the same initiator port.

Serial-Attached SCSI

Serial-attached SCSI (SAS), is a technology that supports SCSI interfaces. SAS can be used in place of SCSI, especially when high performance is needed; in fact, in enterprise environments, SAS is the popular alternative to parallel SCSI. The reason is not farfetched, SAS supports up to 65,535 devices, thanks to expanders, while the newest parallel SCSI standards only allow 16 devices.

SAS vs. SATA - Who is faster?

SAS is a point-to-point technology not subject to resource contention issues common with parallel SCSI. Serial-attached SCSI is an upgrade to the older, almost obsolete parallel ATA (PATA) standard – just like SATA. Also, SAS is backward compatible with SATA-2 and newer SATA standards.

SATA-3 has a speed rating of up to 6Gbps/600MBps – slower than the Ultra640 SCSI standard. SAS is inarguably faster than SATA, but both are newer serial bus standards to replace parallel SCSI in modern-day computing environments.

It is worth noting that while SAS and SATA drives use an SCSI command set, SAS drives cannot connect to a SATA controller and vice versa. SAS is best for servers and workstations while SATA is a decent, less expensive choice for personal computers. SATA is fast for personal computing.

Tip: Learn about VMware data recovery!


SCSI is a very fast interface for heavy transfers and to power heavy appliances, but it is old and getting replaced by newer technologies such as SATA and SAS. SAS is actually based on SCSI technology and is used in enterprise data centers and server environments. SCSI interface is mostly found on computers and storage devices. “Smaller” personal computing devices like mobile phones and media players (MPs) use Universal Serial Bus (USB).


  • What is the SCSI used for?

    The Small Computer System Interface (SCSI) represents a collection of standards designed for the physical connection and data transfer between computers and peripheral equipment, notably utilized with storage devices like hard disk drives.

  • Is SCSI still being used?

    In personal computing, the SCSI interfaces have largely been supplanted by Universal Serial Bus (USB) technology. However, within the enterprise sector, SCSI continues to be employed in server farms for managing hard drive controllers.

  • What is the main use of SCSI controller card?

    SCSI is applicable for various external storage devices, including hard drives, CD/DVD drives, and tape drives. It accommodates a diverse array of device types, with external SCSI devices providing benefits like enhanced data transfer speeds and the capability to link several devices in a daisy-chain setup.

  • What does the SCSI adapter do?

    The SCSI adapter device driver oversees the operation of the SCSI bus without directly managing the SCSI devices themselves. It has the capability to dispatch and accept SCSI commands, though it lacks the functionality to decipher the command contents. Additionally, this lower-level driver facilitates error recovery and maintains logs for issues concerning the SCSI bus and system I/O hardware.

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