Hard Drive Speed

DocumentID: 612299
Revision Date: 29-Feb-96 1:35:26 PM

The information in this document applies to:
WordPerfect® 5.1 for DOS

Problem

Solutions: Description Hard disk drive speed is measured with two different measurements Average access time Transfer rate Explanation Average access time is the average amount of time it takes to move the read/write heads from one point on the disk to another. This time is measured in milliseconds (ms). This access time is drive specific and has little to do with the type of interface card installed. Transfer rate is a measurement of the speed at which the drive and controller can transfer data to memory. The transfer rate is dependent upon the type of controller installed. Different types of controllers transfer at different rates. IDE, SCSI, and ESDI controllers generally transfer data at a higher rate than do RLL or MFM controllers. The hard drive itself also plays into this equation. Some IDE hard drives transfer data at rates nearly double that of other IDE drives. Theoretically, SCSI drives and controllers are the fastest at transferring data. Some controllers have built-in caching which makes them appear even faster. In effect they are much faster because many times they don't actually read the drive, they read the cache memory. Some drives have a "look-ahead buffer" which is similar to a cache, but instead of just caching what has been read, the drive's logic tells it to read information that has not yet been requested and save it in the buffer. The transfer rate is the most important factor in the overall performance of the drive. Average Access Time is merely the time taken to find the information to transfer. Transfer Rate is the speed in which this data can be transferred. Transfer Rate can be compared to a funnel. The narrower the neck of the funnel, the slower the substance moves or flows through the funnel. With a very narrow funnel neck, rapid access times don't improve overall performance very much. 32-Bit Disk Access This is a newer technology available only on a small percentage of systems. Windows has an option to turn on 32-bit disk access. This allows maximum data transfer speed. The "32-bit" designation refers not to the processor, but to the bus. Only a small percentage of computers can support this at this time. These are some EISA and PS/2 machines. However, the computer must have a 32-bit EISA or Micro Channel controller installed. Standard controllers are 16-bit (8-bit on XTs and older systems). Note: The drivers included with Windows only support Western Digital 32-bit hard disk controllers (or compatibles). If the customer's hard disk controller is not 100% Western Digital compatible, this option cannot be used, unless they have drivers from that manufacturer. Warning: The option can be turned on whether it works or not! If customers are unsure about their controller, they SHOULD NOT turn 32-bit access on-- loss of data can result.

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