The physical disk technology that is still considered the traditional storage interface for enterprise servers and offers transmission rates up to 640 MB/sec is the Small Computer System Interface (SCSI).
What is SCSI?
SCSI (pronounced “scuzzy”) stands for Small Computer System Interface. It is a set of standards for physically connecting and transferring data between computers and peripheral devices. The SCSI standards define commands, protocols, electrical interfaces and optical interfaces that allow SCSI devices to communicate with each other.
Some key things to know about SCSI:
- SCSI was introduced in the early 1980s as a parallel interface standard for attaching peripheral devices to computers.
- It allowed for higher transmission speeds compared to the previous parallel standards at the time.
- SCSI uses a bus structure that allows you to daisy chain multiple devices together – up to 8 or 16 depending on the SCSI version.
- Common SCSI devices include hard disk drives, tape drives, CD/DVD drives, printers, and scanners.
- There have been many revisions of the SCSI standard over the years, improving speeds and capabilities.
The fastest SCSI standard currently in common use is SCSI Ultra640. It provides a transfer rate of up to 640 MB/sec, which is where that figure in the question comes from.
Why is SCSI considered the traditional enterprise server storage interface?
There are a few key reasons why SCSI is considered the traditional disk interface for enterprise servers:
- Speed – SCSI offers very high data transfer rates compared to other legacy options like PATA/IDE. The fastest version, Ultra640, can hit 640 MB/sec as mentioned.
- Reliability – SCSI is designed with servers in mind. It has error checking mechanisms and robust connectivity for reliable data transfers.
- External connectivity – SCSI allows you to easily connect external peripherals outside the server box. This makes maintenance and expandability easier.
- Long history – SCSI has been around since the 1980s and is a proven and trusted technology in data centers.
- Parallel interfacing – The parallel SCSI bus allows for multiple devices to communicate simultaneously for high throughput.
For enterprise servers that need fast, reliable connections to storage like hard drives and tape backup units, SCSI has been the top choice for many years. The performance and connectivity of SCSI makes it well suited for servers that need to provide consistent, high-speed access to data.
Brief history of SCSI
Here is a brief overview of major milestones in the history and evolution of SCSI technology:
- 1982 – The first SCSI standard, SCSI-1, is released and provides transfer speeds of 5 MB/sec.
- 1990 – SCSI-2 arrives with faster 10 MB/sec speeds and support for logical unit numbers (LUNs).
- 1995 – SCSI-3 hits 20 MB/sec speeds and brings new command protocols.
- 1997 – Ultra SCSI doubles the clock rate of SCSI-3 for 40 MB/sec transfer rates.
- 1999 – Ultra2 SCSI doubles the transfer rate again to 80 MB/sec.
- 2001 – Ultra160 SCSI increases to 160 MB/sec speeds.
- 2003 – Ultra320 SCSI doubles that again to 320 MB/sec data rates.
- 2005 – Ultra640 SCSI hits 640 MB/sec transfer speed.
- 2008 – SAS-1 serial standard released, marking the beginning of the end for parallel SCSI.
So in summary, SCSI rapidly evolved over its first 20 years to provide ever-faster data transfer speeds to keep up with enterprise server demands. It peaked with Ultra640 at a staggering 640 MB/sec transfer rate before serial connections like SAS ultimately took over in enterprise environments.
SCSI vs SATA vs SAS
It’s helpful to compare SCSI against the other two major hard drive / storage interfaces:
- SATA – Serial ATA. The current standard for consumer hard drives and SSDs. Slower than SCSI but cheaper.
- SAS – Serial Attached SCSI. The serial evolution of SCSI designed for enterprise. Replaced parallel SCSI in the 2000s.
Here’s a quick comparison between the 3 interfaces:
Attribute | SCSI | SATA | SAS |
---|---|---|---|
Release year | 1982 | 2003 | 2004 |
Transfer rate | Up to 640 MB/sec | Up to 600 MB/sec | Up to 12 Gb/sec |
Interface | Parallel | Serial | Serial |
Cable length | Up to 25 m | Up to 1 m | Up to 10 m |
Hot swappable | Yes | No | Yes |
Price | Expensive | Cheap | Moderate |
Primarily used in | Enterprise servers | Consumer PCs | Enterprise servers |
As you can see, SCSI offers excellent performance compared to SATA drives designed for home computers. And while SAS has now overtaken SCSI in enterprise applications, SCSI had a long reign as the fastest hard drive interface available during the 80s, 90s and early 2000s.
Advantages of SCSI
Here are some of the key advantages that made SCSI popular for enterprise servers:
- Speed – SCSI, especially later versions like Ultra160 and Ultra640, offered incredibly fast data transfer speeds for their time.
- Reliability – With robust connectors and shielded, thicker cabling, SCSI provided much greater reliability than consumer IDE cabling.
- External connectivity – SCSI made it easy to attach external devices like tape drives without having to crack open the server box.
- Daisy chaining – Multiple SCSI devices can be daisy chained together on the same bus cable.
- Hot swapping – SCSI devices can be detached and reattached without rebooting the server, providing high uptime.
- Broad device support – SCSI offers excellent compatibility with disks, tapes, printers, scanners and other peripherals.
For mission-critical business computing with little tolerance for downtime, SCSI provided everything enterprise IT teams needed – quick access to large amounts of data, resilient connections, and easy expanding and swapping of hardware. It’s these advantages that made SCSI a data center staple for so many years.
Disadvantages of SCSI
Despite its advantages, SCSI does have some downsides that led to its decline:
- Expensive – SCSI cables, terminators, host adapters and disks are more expensive than consumer SATA or IDE parts.
- Complex – Getting all the SCSI IDs, termination, and cabling right requires training and experience.
- Cable length limits – SCSI parallel cables are limited to only a few meters, requiring devices to be close together.
- Single host – Each SCSI bus can only have one SCSI host adapter card, limiting expandability.
- Difficult troubleshooting – With data on parallel cables, problems are hard to diagnose compared to point-to-point serial connections.
Ultimately, the narrow and short cable runs required by parallel SCSI limited its ability to serve large storage arrays spread over distance. The introduction of serial interfaces like SATA and SAS addressed many of these disadvantages while retaining the fast performance.
The decline of SCSI
Following its peak in the 1990s and early 2000s, the popularity of SCSI for enterprise servers has declined over the past 15+ years. There are a few key reasons for this:
- The rise of high-speed serial interfaces like SATA and SAS that could surpass SCSI performance.
- Limitations of parallel SCSI cables for reaching distant storage devices.
- Continued cost declines of SATA making it viable for more enterprise use cases.
- Development of new very high-speed network storage technologies like FCoE, iSCSI, and NVMe over Fabrics.
- Increasing SSD adoption in servers, which favor SATA/SAS/M.2 connections over legacy SCSI.
Around 2008, SAS began to rapidly replace SCSI in enterprise servers given its superior cabling reach, theoretical speed, and roadmap for future development. Most data centers these days have transitioned the majority of their storage infrastructure to SAS disks and SSDs.
SCSI technology is now mainly found in legacy enterprise hardware that has not yet been refreshed. But for new server and storage deployments, SAS and SATA are now the interface of choice.
SCSI standards evolution
Here is a summary of the major SCSI standard evolutions over the years:
Standard | Year | Speed | Key Features |
---|---|---|---|
SCSI-1 | 1986 | 5 MB/sec | First SCSI standard |
SCSI-2 | 1990 | 10 MB/sec | Introduced SCSI ID system |
SCSI-3 | 1992 | 20 MB/sec | Improved protocols |
Fast SCSI | 1994 | 20 MB/sec | Doubled clock rate of SCSI-2 |
Ultra SCSI | 1994 | 40 MB/sec | Doubled Fast SCSI speed |
Wide SCSI | 1995 | 40 MB/sec | Increased bus width to 32 bits |
Fast Wide SCSI | 1996 | 80 MB/sec | Combines Fast and Wide |
Ultra2 SCSI | 1998 | 80 MB/sec | Doubles Ultra SCSI speed |
Ultra160 SCSI | 1999 | 160 MB/sec | Doubles Ultra2 speed |
Ultra320 SCSI | 2001 | 320 MB/sec | Doubles Ultra160 speed |
Ultra640 SCSI | 2003 | 640 MB/sec | Doubles Ultra320 speed |
As you can see, there was rapid advancement from the initial 5 MB/sec transfer rate in 1986 to an incredible 640 MB/sec by 2003 – a 128x improvement in speed! This demonstrates how critical it was for SCSI to continue evolving to meet data center storage demands.
The future of SCSI
The future of SCSI itself is quite limited, given that serial connections like SAS and SATA have almost entirely replaced SCSI for new server deployments.
There is little need or incentive for vendors to invest in developing new SCSI standards given the constraints of parallel SCSI cabling and the wide industry adoption of SAS.
Existing legacy SCSI hardware will continue to be used in older data centers until it is refreshed, but no major advancements in the base SCSI standard are expected moving forward.
That said, the SCSI command set does live on in SAS, which uses a SCSI command protocol over its serial interface. So in that sense, the legacy of SCSI continues, even if the physical interface itself sees declining use.
Conclusion
SCSI enjoyed decades of dominance as the storage interface of choice for enterprise servers. Its excellent performance, robust connectivity, and simple expandability via daisy chaining made it perfect for connecting hard drives and tape devices.
Reaching speeds up to 640MB/sec, SCSI drove the rapid growth in data center storage capacity through the 80s, 90s and early 2000s.
However, physical limitations of parallel SCSI signaling eventually gave way to new serial attached standards like SAS and SATA. This brought faster speeds, simplified cabling, and lower costs – essentially making SCSI obsolete.
While SCSI itself is fading away, the SCSI command protocol continues to thrive as the basis for SAS connectivity that dominates modern data centers. So while SCSI is no longer common in enterprise environments, its lineage and legacy live on in next generation storage networking standards.