Hard Disk

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Hard Disk

The Physical Characteristics of All Disks

The hard disk has one or more metal platters coated top and bottom with a magnetic material similar to the coating on a VCR magnetic tape. In the VCR the tape moves by a fixed recording and sensing device (the "head").  In a disk, the recording head is on a movable metal support called the "arm". Information is recorded onto bands of the disk surface that form concentric circles. The circle closest to the outside is much bigger than the circle closest to the center. Since each metal platter has a top and bottom surface, there are at least two magnetic circles for each size and location. However, a disk may have as many as five platters, producing ten of these identical circles at the same distance out from the center. There is a separate magnetic read/write head for each disk surface. With five platters there are ten heads. They are all fixed on metal "arms" that can move the head from the innermost to the outermost circular position on the disk surfaces. The arms all move together, so if there are 10 disk surfaces the read/write heads are in the same location on all 10 surfaces simultaneously. One circle on a disk surface, equivalent to one position of the read/write head on the end of the arm is called a track. All of the tracks on all of the surfaces that correspond to the same arm position used to be called a cylinder. However, since the outer circles are bigger they can hold more data, and modern disks hide the actual location of the arm so that no operating system can determine exactly where it is. So today the term "cylinder" is typically used to refer to a certain number of megabytes of disk space that can be assigned to a partition (a disk letter in Windows). To find some particular piece of data, the disk must move the arms to the position where it is located. This is called seek. Then it must wait for the data on the disk surface to rotate around until the start of the data comes under the read head. This is called rotational latency. There are three types of disks in common use:

• Desktop 3.5" disks rotate at 7200 RPM. Their average seek time (to move the arm) is around 9 milliseconds. • Laptop 2.5" disks often rotate at 5400 RPM and have a longer seek time than desktop disks. They use a lot less power which allows longer battery life, but at the cost of slower performance. • Enterprise disks (which can be 3.5 but increasingly are 2.5") rotate at 10,000 or 15,000 RPM and have an average seek time of 4.5 milliseconds. This means they are twice as fast as desktop disks, but they are also more expensive and hold much less data. Desktop disks typically hold 250 to 500 gigabytes, although 750G disks are available and 1 terabyte disks are coming. Laptop disks are available in up to 250 gigabyte sizes, though 80 to 160G is more common. Enterprise disks come in 150 and 300G sizes. Desktop and laptop disks come with parallel ATA and SATA connectors. Newer systems use SATA, so parallel ATA is needed only to replace or upgrade an older system. Enterprise disks typically use SAS (Serial Attached SCSI). Western Digital produces one disk family called the Raptor. This is an Enterprise class disk with a SATA connector sold to desktop users. It is by far the best performer for your primary (operating system) disk, but it does have a higher cost. However, the dirty little secret is that 99% of ordinary Windows, Office, Web Browsing, and Email reading doesn't use much CPU or video and will not be improved by the stuff you usually spend money on. If instead, you bought the absolutely lowest cost main board, CPU, memory, and video and replaced the ordinary hard drive with a Raptor your typical user would find the system boots twice as fast and runs faster than some pimped out gaming rig costing five times as much but equipped with ordinary disks. It requires more power to rotate a disk faster and to move the arms (seek) faster. At the highest speeds, limitations...
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