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Form Factors
Hard Disks are magnetic media - electrons are arranged on strong circular metal plates called platters. As technology has grown more and more complex, the Arial density level increases, allowing for larger amounts of storage. There is an end to conventional storage in sight though - the Super Paramagnetic Limit, once this limit is passed, electrons begin to disperse and interact with each other in ways that corrupts all data on the disk.
A single platter is not enough for the storage needs of today, and it never really was. The super-paramagnetic limit restricts the amount of data each platter holds, so hard disk companies need to use common tricks to get around it: using more than one platter, or increasing the diameter of a platter. Because of these tricks, there are two distinctly different hard drive form factors:
3.5" - The same shape as a 3.5" floppy drive.
5.25"- The same shape as a CD-ROM Drive.
For Notebooks, the sizes need to be even smaller. The most common size is 2.5 inches, but some are 1 inch in size! IBM's new matchbox drive is literally the same size as a match box, with a single platter supporting in excess of 300 megabytes.
The problem with increasing platter size are threefold. Not only are there PC Case size limitations, but larger platters need more powerful motors, and are more susceptible to vibration damage. With the effects of gravity, uniform flatness is also a serious issue.
Capacity
The largest consumer level drive today has 19.2 Gigabytes of space. These totals can be deceptive though - the actual formatted and usable storage area is often less than what is advertised on the boxes of today's hard disks. It's not that the manufactures are outright lying, instead they are taking advantage of the fact that there's no standard set for how to describe a drives storage capacity. Here's and explanation snipped from a hard drive review at Zdnet
This results from a definitional difference among the terms kilobyte (K), megabyte (MB), and gigabyte (GB). In short, here we use the base-two definition favored by most of the computer industry and used within Windows itself, whereas hard drive vendors favor the base-10 definitions. With the base-two definition, a kilobyte equals 1,024 (210) bytes; a megabyte totals 1,048,576 (220) bytes, or 1,024 kilobytes; and a gigabyte equals 1,073,741,824 (230) bytes, or 1,024 megabytes. With the base-10 definition used by storage companies, a kilobyte equals 1,000 bytes, a megabyte equals 1,000,000 bytes, and a gigabyte equals 1,000,000,000 bytes.
Put another way, to a hard drive manufacturer, a drive that holds 6,400,000 bytes of data holds 6.4GB; to software that uses the base-two definition--including CHKDSK, and portions of Windows 95 and Windows 98--the same drive holds 6GB of data, or 6,104MB.
So, be prepared when you format that new 6.4GB drive and find only 6GB of usable storage space. Isn't marketing wonderful?
Rotations per Minute (RPM)
The platters in a hard disk are connected in the middle by a spindle and motor. The motor spins the platters at a specific rate, known as RPM. Higher speeds allow data to be read/written much faster, along with reducing seek time. However, it's also a proportional increase in heat. The following list shows the typical RPM of today's hard disks:
3600 RPM (Pre-IDE)
5200 RPM (IDE)
5400 RPM (IDE/SCSI)
7200 RPM (IDE/SCSI)
10000 RPM (SCSI)
At higher speeds there is more stress and heat on the platters and electronics, 10000 RPM seems to be the current maximum. With the advent of Ultra-ATA 66, manufactures are working hard to release 10000 RPM IDE drives. They promise even lower access times and higher transfer rates, but are still not nearly as effective as their SCSI counterparts in terms of CPU Utilization and thoroughput."
The Hard Disk Cache
A small amount of memory incorporated into the hard disk electronics to accelerate read/write times. When the computer requests data from the hard disk if that data is in the cache, there is a performance boost directly related to the speed of the cache.
A visual representation: Imagine you are assembling something and have a box of different size screws, you need eight identical screws for this step in the project. When you look into the box (hard disk) for the first screw you happen to see 5 of the eight that you need so you grab the five that you see and put them onto the table (Cache) now when you need the next screw you won't have to dig into the box, instead you grab one from the table (Cache). Much faster than digging into the box each time.
The hard disk cache controller works in a similar manner except instead of seeing the data needed the cache controller guesses and reads a small amount of data just before and just after the data it was requested. When the program requests more data the hard drive first looks into the cache to see if the data is there.
The Hard Disk Cache is also used as a queue - if there is more than one operation to carry out, the instructions can be left in the Cache.
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