Why is a hard disk drive so slow compared to a solid state drive and why don't they just get rid of the hard disk drive, and store everything in solid state memory? Since 2006 computer manufacturers have been producing Solid State drive based PCs.
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Solid State Drive (SSD) Basics

When a computer runs an application, or even the operating system, it first copies the code for the application from it's hard disk drive to its main memory, then it runs the application from main memory. The reason for this is because a hard disk drive is a relatively slow serially accessed mechanical device, while main memory is constructed of solid state devices called Random Access Memory (RAM).

A hard disk is constructed of a number of platters in a stack. The surfaces of the platters are covered with a magnetic coating. Electromagnetic read/write heads are positioned above and below each platter. As the platters spin, the drive heads move in toward the center surface and out toward the edge. In this way, the drive heads can reach the entire surface of each platter.

Data is stored on a hard disk in arc shaped paths called clusters. In order to access a particular piece of data, the read/write heads must move to the proper position on the platters, and then wait for the platter to spin until the proper cluster is under the read/write heads. Then the clusters containing the code for the data being requested may be read from the hard disk drive.

Unfortunately, unless the disk has been recently defragmented, the clusters containing the code for any particular application will not be in contiguous clusters. In this case, in order to access the entire code for an application, the read/write heads will be required to move in and out across the platter, and wait for each individual group of clusters containing the code for the application to come under the read/write heads. In computer time, this is a very slow operation.

To read data from the computer's main memory RAM, the computer's solid state bus interface unit places the addresses of the application's data on the solid state RAM's address port, and the RAM places the data at those addresses on the data bus. Since RAM is "randomly accessed" rather than serially accessed, the concept of fragmentation does not apply. Thus data can be accessed from solid state RAM much faster than from a mechanical hard drive.

You might ask, then why don't they just get rid of the hard disk drive, and store everything in solid state memory? Unfortunately, RAM is volatile, in other words when you shut off the power the data it contains is lost. There is a type of solid state memory called "flash memory" that is is a non-volatile, the type used in memory cards and USB flash drives, but it's too expensive to used for the amount of storage required to replace a hard disk drive. Too expensive that is - until now.

• The typical access time for a hard disk drive is around 5 milliseconds, while the typical access time for a solid state drive is around 0.1 millisecond. This makes the solid state drive about 50 times faster than a hard disk drive.

Lenovo Laptop with 64 GB Solid State Drive
Lenovo Laptop with 64 GB Solid State Drive

Since 2006 computer manufacturers have been producing Solid State drive based PCs. The ASUS Eee makes a laptop that uses a 20 GB SSD. Lenovo makes a laptop that uses a 64 GB SSD. Apple makes a laptop that has a 128 GB SSD. Even the computer developed by the One Laptop Per Child (OLPC) association, uses an SSD rather than a hard drive.

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