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Solid state disk (SSD, also called solid state drive) is a data storage device that uses memory chips, such as SDRAMs, to store data, instead of the spinning platters found in conventional hard disk drives. While not technically "disks" in any sense, these devices are so named because they are typically used as replacements for disk drives in situations where space, power supply, or ruggedness concerns would make conventional drives impractical.
SDRAM-based
The term "SSD" is used for two different kinds of products. The first, SSDs based on fast, volatile memory such as SDRAM, are categorized by extremely fast data access, less than 0.01 milliseconds (over 250 times faster than the fastest hard drives in 2004) and are used primarily to accelerate applications that would otherwise be held back by the latency of disk drives.
Since these SSDs use volatile memory, they typically incorporate internal battery and backup disk systems to ensure data persistence. If power is lost for whatever reason, the battery would keep the unit powered long enough to copy all data from random access memory (RAM) to backup disk. Upon the restoration of power, data is copied back from backup disk to RAM and the SSD resumes normal operation.
These devices are especially useful on a computer which already has the maximum amount of RAM. For example, some x86 architectures have a 4 GB limit, but this can effectively be extended by putting the swap file on a SSD. These SSD do not provide as fast storage as main RAM because of the bandwidth bottleneck of the bus they connect to.
FLASH-based
The second type of SSD uses flash memory to store data. These products, usually the same size as conventional storage are typically used as low power, rugged replacements for hard drives, especially in installations exposed to extreme conditions. To avoid confusion with the first type, these disks are generally referred to as "flash disks." Occasionally, the term is used to include keydrives, since they are technically solid-state and RAM-based. The flash memory cells tend to fail after around a million writes which made early devices unsuitable for storage which is often updated in place, such as swap files. To address this problem, some SSD vendors have introduced wear-levelling techniques that track usage and transparently relocate the data in highly utilized extents of storage to extents that have been less utilized.
Another use for flash drives is running lightweight operating systems designed specifically for turning general-purpose PCs into network appliances comparable to more expensive routers and firewalls. In this situation a write protected flash drive containing the whole operating system is booted from. A similar system could boot from CD, floppy disk or a traditional hard drive but flash memory is a good choice because of very low power consumption and failure rate.
Both types of solid state disk are relatively expensive and low-capacity compared to hard drives. Their unique ability to solve specific problems of data storage, however, has created a small but consistent demand for these products since the 1970s.
At the 2006 CeBIT show, Samsung Electronics of South Korea showed a protype laptop computer (a modified Samsung Q30) featuring a 32GB Flash Disk, with a reported cost of US$3,700. Samsung announced on 23rd May 2006 that its first products using the 32GB SSD will be an UMPC and a notebook PC, both available June 2006 to a Korean-only market.
See also
- RAM disk - virtual SSD drive which uses a segment of a computer's main memory as secondary storage.
External references
"Solid state disk"