STORAGE

Hard Disk Technologies

There are now three primary types of disk interface in everyday PC use SCSI, IDE and serial ATA but there are variants of each. This document is only intended as a guide to the types of interface and further reading is advised if you wish to learn more.

IDE / EIDE (you may also encounter related terms such as ATA66 or ATA100 amongst others.)

Primarily used in desktop PCs, laptops and low-end server systems IDE is limited to two devices per channel – a master and a slave. Most PCs have two IDE channels integrated into the motherboard. It is the most popular (and cheapest) of the current disk types and is used extensively for hard disks, CD-ROMs, rewriters, DVDs, ZIP drives and low-end tape systems. Data transfer is measured in megabits per second (ATA100 is 100 etc) and is per channel - thus to achieve 100MBit data transfer from one device to another (or close to), each device would need to be on separate channels. The system uses a standard 40 pin flat connector and miniature version for laptops.

SCSI / SCSI-2 / SCSI-Wide

Used in systems where more devices are likely to be required and better performance is essential, a SCSI interface will offer 7 (or 14 in SCSI-2/Wide) devices but the controller card is one of these. Multiple controllers can be used and each 'chain' must be terminated with a resistor pack to prevent degradation of signal quality (a bit like ghosting on a TV!) – most devices have this resistor pack integrated and it is enable using a small metal link or 'jumper'. Although generally regarded as vastly superior to IDE, many of the performance advantages have been 'caught-up' by IDE and accordingly the price variance is less than it once was - but still more than IDE. There are various connectors and speeds - the fastest is currently Ultra320 (320Mbits/second). As well as disks, higher capacity tape devices, scanners and high end optical drives tend to use SCSI.

Serial ATA

This is a relatively new format, designed to replace IDE - it uses fewer conductors in the connection cable (like USB) and is designed to allow more devices (like SCSI) and higher speeds. 133 and 166 MBits are currently available. These drives need a special Serial ATA controller card and some motherboards are now including this interface.

Optical storage There are two types of optical storage likely to be encountered in small business IT systems - CD and DVD. CD-ROMs (read only) are now commonplace in PCs and DVD-ROMs are also becoming more popular due to home markets as DVD Videos can be played on PCs. Breakthroughs in media technology have allowed writeable and re-writable disks to be constructed for both formats and these are now becoming more widespread - particularly the CDRW/DVD Combo drive (Read CD, Write CD, Rewrite CD and Read DVD). DVD Writers are falling in price and will become more widespread. CDs can store up to 700MB and DVDs 4.7GB although new 'blue laser' HD-DVD formats are in the pipeline, promising 15GB (or 30GB double-sided).

Tape storage

There are many historical tape formats, but the most common families now are DDS (Digital Data Storage) Traven and DLT (Ultrium).

DDS offers the lowest tape cost but drives are not usually the cheapest. Data transfer rates are good and capacities are as follows:- DDS3=12/24GB DDS4=20/40GB and DDS5 (or DAT72)=36/72GB. The first value is the tapes 'native' capacity and the second the capacity if the data is compressed. Compression is common for tapes but does take longer to write and read - it can be done by the drive (hardware compression) or the backup software (software compression). If you have a lot of office documents, web pages, databases etc the compression rate will probably be better than 50% but on the other hand if the content is media (MP3s, MPEGs or AVIs etc) or zipped archives then the compression will be much lower - perhaps even zero! (These file types are already compressed)

ZIP Drives, Floppy Disks, LS-120 Drives and Flash Drives

These formats are all useful for short-term backups and moving data between PCs or offices but with the growth of multimedia and broadband, their use is becoming increasingly limited. The exception to this is perhaps the USB Flash Drive which is essentially a memory chip with a USB interface that can store large amounts of data indefinitely. These devices are cheap (under £20 for 64MB) relatively quick and very portable!