Is it scuzzy enough?

CHIPS, Summer, 2004 by Patrick G. Koehler

What is scuzzy? Scuzzy refers to the small computer system Interface (SCSI). SCSI is a parallel interface standard used to connect external hardware such as tape drives, removable drives, external CD-ROMs, etc., to personal computers, Unix systems and Apple Macintosh computers.

SCSI popularity was stifled by the onslaught of new technologies such as Firewire (IEEE 1394) and Universal Serial Bus (USB), but it continues to evolve and is the preferred choice for large servers or systems that support many peripheral devices. SCSI's backward compatibility and legacy support are the principal reasons for its survivability. Powerful computer operating systems using Microsoft Windows, OS/2 and Unix support multithreading and multitasking and helped SCSI devices gain in popularity.

SCSI features support many different components, allow faster data transfer and provide connection for multiple internal and external peripheral devices. Let's examine SCSI standards, characteristics and tips for buying SCSI peripherals. There are three SCSI standards: SCSI-1, SCSI-2 and SCSI-3. SCSI-2 is still in use, but SCSI-3 is the current standard.

The Shugart Associates System Interface (SASI), a predecessor to SCSI, was developed in 1979. The American National Standards Institute (ANSI) ratified the first standard in 1986 calling it SCSI-1. SCSI-1 did not share a common standard which caused equipment incompatibilities. The first design had a narrow 8-bit bus, slow speed and short cable length. SCSI-1 included a single-ended (SE) transmission supported by a passive termination.

SCSI-2 was the revised, compatible standard, ANSI approved in 1994. A common command set was established so that a Seagate SCSI drive could easily work with an Adaptec SCSI or Western Digital controller. SCSI-2 was a definitive enhancement over SCSI-1. SCSI-2 featured a wider data bus doubling in size from 8- to 16-bit supporting 16 devices. The SCSI adapter takes up one device ID number. A Fast-Wide SCSI-2 can support up to 15 devices because the adapter requires one SCSI ID. SCSI-2 also introduced differential signaling methods: High Voltage Differential (HVD) and Low Voltage Differential (LVD).

HVD and LVD signaling methods increase data transfer speed and lengthen the signal on the SCSI cable. Technology enhancements further evolved SCSI-2 devices by using active and forced perfect termination (FPT) methods. The update added a new command set to support tape drives, CD-ROMs and CDR/RWs. SCSI-2 includes command queuing to allow a server or system to handle multiple requests at the same time, which increases performance for server farms, clusters and Storage Area Networks (SANs). Ultra SCSI-2 and Wide Ultra SCSI-2 increase data performance.

Internet SCSI (iSCSI) transmits data over Internet Protocol (IP). It is a protocol-based standard ratified by the Internet Engineering Task Force (IETF). iSCSI brings a new approach to data storage by using Host Bus Adapters (HBA) that appear to be like a network interface card (NIC) on the network that has its own IP address to communicate. The server then transfers data to the iSCSI device. This transfer is transparent to the user. iSCSI uses the common Ethernet infrastructure to communicate with the server, and it is fl exible and easy to maintain.

iSCSI works with the new 10 Gigabit Ethernet standard to perform high speed data transfers, which are much faster than the typical NAS (network-attached storage) or SAN device. There is a security concern with iSCSI. As with other SCSI devices, encryption was not built-in, so a third party device, software or operating system configuration may be required.

Serial Attached SCSI, or SAS, is a recent standard that takes SCSI to new heights with faster data transfer rates that can travel greater distances. SAS brings to the table point-to-point topology using dedicated disk connections with scalable throughput. SAS performance has risen to 3.0 GBps (300 MBps) in 2004, doubling the 1.5 GBps (150 MBps) throughput available for Serial ATA (Advanced Technology Attachment) in 2002. It allows smaller cables for improved air flow while providing fewer signals for high density routing. SAS has good disk and backplane interoperability offering a wide range of deployment options. SAS is less expensive with the added benefit of ATA compatibility, which simplifies the upgrade process and keeps maintenance costs down.

Here are a few tips for selecting SCSI components.

* Signaling: Select the correct signaling, considering the distance between your controller and the target device(s). There are three types of signaling methods: SE, HVD and LVD. SE has a much shorter signal range (10 feet) than LVD (40 feet) or HVD (80 feet). The signaling method used is affected by the data bus width and whether it is 8- or 16-bit. The narrow data bus limits the cable length.

An important consideration in picking a SCSI adapter and devices is how long the cable has to be to connect all your SCSI internal and external devices. Length is affected by the data bus width, SCSI standard and whether you are connecting two or more devices. HVD signaling assures of you maximum cable length.