Solid State Disk (SSD)

A solid state disk (SSD) - is electrically, mechanically and software compatible with a conventional (magnetic) hard disk.

A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. Unlike flash-based memory cards and USB flash drives, a SSD emulates a hard disk drive interface, thus easily replacing it in most applications. An SSD using SRAM or DRAM (instead of flash memory) is often called a RAM-drive.

The original usage of the term solid-state (from solid-state physics) refers to the use of semiconductor devices rather than electron tubes, but has in this context been adopted to distinguish solid-state electronics from electromechanical devices as well. With no moving parts, solid-state drives are inherently less fragile than hard disks and therefore also silent (unless a cooling fan is used); as there are no mechanical delays, they usually enjoy low access time and latency.

The difference is that the storage medium is not magnetic (like a hard disk) or optical (like a CD) but solid state semiconductor such as battery backed RAM, EPROM or other electrically erasable RAMlike chip such as flash.

This provides faster access time than a hard disk, because the SSD data can be randomly accessed in the same time whatever the storage location. The SSD access time does not depend on a read/write interface head synchronising with a data sector on a rotating disk. The SSD also provides greater physical resilience to physical vibration, shock and extreme temperature fluctuations. The only downside is a higher cost per megabyte of storage - although in some applications the higher reliability of SSDs makes them cheaper to own than replacing multiple failing hard disks.

When the storage capacity needed by the application is small (as in some embedded systems) the SSD can actually be cheaper to buy because hard disk oems no longer make low capacity drives. Also in enterprise server acceleration applications - the benefit of the SSD is that it reduces the number of servers needed compared to using hard disk based RAID on its own.

Historically RAM based SSDs were faster than flash based products - but in recent years the performance of the fastest flash SSDs has been more than fast enough to replace RAM based systems in many server acceleration applications.

Both types of SSDs are available in a wide range of form factors and supporting traditional disk interfaces. A complete list of manufacturers with tables by form factor, technology type and interface type is updated in real-time in the Solid State Disks Buyers Guide

The reasons that users might benefit from buying SSDs are listed in the SSD Market Adoption Model

Take the case of SSD speedup in servers. One way of thinking about this concept in computer architecture is - SSD CPU Equivalency. For a wide range of applications if you take a black box approach and analyze the overall application performance of a computer system - you would not know whether that system had more CPUs with hard disks or less CPUs with more SSDs.