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The Storage Guide

The Interface (EIDE / SCSI)

Currently there are 2 different common interfaces: EIDE and SCSI. You will find an EIDE controllers integrated with the motherboard and that EIDE harddisks are much cheaper than SCSI drives. For SCSI you need an extra controller, because there aren't a lot of motherboards with integrated SCSI controllers. Together with the higher price of a SCSI disk a SCSI system is more expensive than EIDE.

The EIDE interface has a primary and a secondary channel that will connect to two devices each, for a total of four. That could be a harddisk, CD-ROM or disk changers. Lately there have been tape backups with EIDE connectors, but you need special backup software.

Scanners for example aren't available with EIDE interface, only with SCSI. You can connect up to 7 devices to a SCSI bus or 15 devices to a Wide SCSI. In a standard environment, the performance of single harddisk won't improve much from the SCSI interface. Rather, the power of SCSI is that several devices can use the bus at the same time, not using the bus while they don't need it. So, we see the best benefit from SCSI when several devices are all used on the same bus.

On one EIDE channel, the 2 devices have to take turns controlling the bus. If there is a harddisk and a CD-ROM on the same channel, the harddisk has to wait until a request to the CD-ROM has finished. Because CD-ROM's are relatively slow, there is a degradation of performance. That's why everbody tells you to connect the CD-ROM to the secondary channel and your harddisk to the primary. The primary and secondary channels work more or less independently of one another (it's a matter of the EIDE controller chip).

The SCSI interface comes in several types. 8 bit (50 wire data cabel) or 16 bit (68 wire data cable, Wide SCSI). The clock can be 5 MHz (SCSI 1), 10 MHz (Fast SCSI), 20 MHz (Fast-20 or Ultra SCSI) or 40 MHz (Ultra-2 SCSI).

Possible theoretcial transfer rates of the SCSI bus
SCSI bus clock8 bit(50 wire data cable)16 bit(68 wire data cable, Wide SCSI)
5 MHz (SCSI 1)5 MBytes/sNA
10 MHz (Fast SCSI, SCSI II)10 MByte/s20 MByte/s
20 MHz (Fast-20, Ultra SCSI)20 MByte/s40 MByte/s
40 MHz (Fast-40, Ultra-2 SCSI)40 MByte/s80 MByte/s

The theoretical transfer rate of EIDE is up to 16.6 MByte/s in PIO mode 4 or multi DMA mode 2 (soon 33.3 MByte/s ) with all the problems you may have already faced. However, today's CD-ROM's often use PIO mode 3, while older device use PIO mode 0 to 2. Sometimes devices lie about the PIO mode they support. There are harddisks that say they are able to use PIO mode 2 but they only work reliably in PIO mode 1! Whenever you get errors accessing your harddisk, try to lower the PIO mode first!

Possible theoretcial transferrates of the IDE bus (ATA)
single word DMA 02.1 MByte/s
PIO mode 03.3 MByte/s
single word DMA 1, multi word DMA 04.2 MByte/s
PIO mode 15.2 MByte/s
PIO mode 2, single word DMA 28.3 MByte/s
Possible theoretcial transferrates of the EIDE bus (ATA-2)
PIO mode 311.1 MByte/s
multi word DMA 113.3 MByte/s
PIO mode 4, multi word DMA 216.6 MByte/s
Possible transferrates of Ultra-ATA (Ultra DMA/33)
multi word DMA 333.3 MByte/s

It is not only the interface transfer rate that determines how fast a harddisk is. How fast the data can be written or read from the media, e.g. data density and rotation speed is more important. The fastest interface can't do anything faster than the 'inner values' of a harddisk are capable of. Today, most harddisks are still under 10 MByte/s transfer rate physically. A faster interface is advantageous on when data is read from or written to the cache in a multitasking environment with several devices accessed simultaneously.

Multitasking environments especially benefit from SCSI, since simultanoues access occurs frequently. If you have a server or are working with large files like audio, video or disk-intense applications, you will benefit more from SCSI than EIDE. There are three reasons for this:

  • All modern operating systems now supports SCSI very well. Windows 3.x didn't!
  • Busmastering really works better with a SCSI busmastering controller.
  • The fastest harddisks with the best performance are SCSI.

If you need large capacities and the highest transfer rates available on the market you need SCSI. This is not because EIDE is incapable of this, it's because of the market. High-end disks with high capacities and high performance are intended to be used in servers and aren't build with EIDE interface. At the moment, EIDE disks are only built with up to a 5 Gigabyte capacity (there is a problem with a 4 GB barrier with some BIOS's again and for drives bigger than 8 GB you need a new BIOS that supports the INT 13 functions AH=41h bis 49h) and transfer rates of about 9 MBytes/s. If you need more, you'll have to use SCSI. Also, SCSI harddisks have larger cache RAM than EIDE harddisks.

Performance, Some Thoughts

You need to know how a slow or fast harddisk affects your overall system performance in a standard enviroment. If your operating system isn't constantly swapping (e.g. you have enough memory) the speed of a harddisk is only a small part of a well balanced system. Let's say you have a harddisk that has 30% better performance than another older one; the benefit for standard applications would be from 2% up to 18%. Sometimes, you want or need the fastest components available. Other times, more capacity and reliability is needed.

There are several programs available that test the performance of a harddisk. Some are crap, others are good. In any case, if you have one, you get numbers that tell you something. But do you have a point of comparison? Different benchmarks mean different numbers. Different environments mean different numbers. Modern benchmarks are independent from existing data on the harddisk (only read performance testing can be done). But a benchmark could be affected by several things:

  • To which channel is the harddisk connected
  • Is the harddisk alone or together with other devices connected to the controller
  • Under which operating system is the harddisk tested and used
  • Which drivers are loaded or not loaded.
  • Testing at Monday or Friday etc.