What is a SCSI Hard Drive?

The SCSI hard drive is mainly used in a 24x7 environment where reliability and speed is important. SCSI drives cost more, not just because they’re fast, but because they’ve been designed to be durable and dependable. The mechanical components have been engineered to survive in a data center environment that places a constant demand on the drive.

SCSI stands for Small Computer Systems Interface and it is affectionately pronounced as “scuzzy.” SCSI drives are often used in servers and RAID controllers. The disks are capable of handling overlapping I/O commands. The ability to do overlapping commands means that the drive can accept a new command while there are still other commands in progress. The typical SCSI disk can handle hundreds of outstanding overlapping commands. This saves much CPU time because the CPU is able to do other tasks during the time between I/O start and I/O completion.

SCSI vs SATA

The performance and reliability of the SCSI hard drive comes at a price. Not surprisingly, a SCSI hard disk can cost 3 to 4 times more than a similarly sized SATA hard drive. While SCSI drives are still faster, the performance of SATA disks is starting to come close to that of SCSI. To save money, many people have considered using SATA drives to replace the more expensive SCSI components.

SATA drives are designed to be used in single user desktop computers. Because these types of computers generally receive only light usage, the drives are not built to run a heavy workload 24 hours a day. All of the mechanical components of a SATA drive, the spindle, the bearings, the actuator arm are built of lesser quality materials to better compete in a low cost market. Having said that, the quality of SATA drives really has been improving. Seagate now offers a 5-year warranty on some of their SATA drives.

Where SCSI really shines is in the corporate data center. SCSI is designed to handle the complete transfer without the aid of the CPU. This enables the computer be more responsive to users. If you want to use SATA drives to replace SCSI disks doing heavy I/O loads, you will also need a faster computer.

Serial Attached SCSI (SAS)

SAS is the latest and best that SCSI technology has to offer. This technology combines a mixture features from SCSI, SATA, and Fibre Channel. Because SAS supports legacy SCSI software, it provides a cost effective migration path for users with lots of parallel SCSI hardware. Most noticeably, SAS has a smaller more streamlined connector. But more than that, it provides a roadmap of improvement for years to come. If you're going to install a SCSI hard drive in your desktop computer, a SAS drive is your best choice.

iSCSI

The iSCSI standard is used for connecting SCSI disks over an Ethernet network. Since it uses the ubiquitous Ethernet, it holds the promise of low-cost centralized storage. Unfortunately, the delays inherent in the Ethernet protocol may be too much of a drag on data transfer speeds.

Because of security concerns, it is not a good idea to use iSCSI on an open Ethernet. The iSCSI disks should only be used in a private network, such as a SAN.

SCSI Standard

The original SCSI standard, or SCSI-1, became official in 1986. The SCSI-2 standard appeared in 1994, and SCSI-3 is the current working standard. The group that works on this standard is called T10. You can get free SCSI Standard Working Drafts on the T10 website.

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