SSD(Solid State Disk)란?

A solid state disk (SSD, also called solid state drive) is a data storage device that uses non-volatile memory such as flash, or volatile memory such as SDRAM, 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 conventional drives are impractical.


A typical RAM-based SSD.

A flash SSD in standard 2.5-inch form factor.SSDs based on volatile memory such as SDRAM and are categorized by 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.



DRAM-based SSDs 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.



However, most SSD manufacturers use nonvolatile flash memory to create more rugged and compact alternatives to DRAM-based SSDs. These flash memory-based SSDs, also known as flash disks, do not require batteries, allowing makers to replicate standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, nonvolatility allows flash SSDs to retain memory even during sudden power outages, ensuring data retrievability. Just like DRAM SSDs, flash SSDs are extremely fast since these devices have no moving parts, eliminating seek time, latency and other electro-mechanical delays inherent in conventional disk drives.


Solid state disks 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, but would still provide a performance increase over placing the swap file on a traditional hard disk drive.



DRAM based SSDs may also work like buffer cache mechanism. When ever a data is written to the memory corresponding block in the memory is marked as dirty block and all dirty blocks can be flushed to the actual hard drive based on following two types. 1. Time (like every 10 seconds, flush all dirty data), 2. Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data).



Flash based SSDs have several unique advantages:
– Faster read time – In some cases, twice or more than that of the fastest hard drives.
– Low read and write latency (seek) time, hundreds of times faster than a mechanical disk.
– Faster boot time – Result of the faster read and especially seek time.
– Lower power consumption – no mechanical parts result in less power consumption.
– No noise – Lack of mechanical parts makes the SSD completely silent.
– Better reliability – Lack of mechanical parts result in less wear and tear. In the past, Flash based SSDs were limited in the number of read/write cycles, but modern Flash technology and error correction allow Flash SSDs to operate several years without fail (some manufacturers claim as many as 10 years).
– Security – allowing a very quick “wipe” of all data stored.
– Deterministic performance – unlike mechanical hard drives, performance of SSDs is constant and deterministic across the entire storage. “Seek” time is constant.

Flash based SSDs also have several disadvantages:
– Price – As of late 2006 Flash prices are still considerably higher per Gigabyte than those of comparable conventional hard drives (around $25 per GB).
– Slower write time – Flash has a slower write time than high-end hard drives (around 18 MB/s compared to over 50 MB/s for hard drives), though this varies considerably based on the make and model.
– Lower reliability – Flash based SSDs have relatively limited read/write cycles compared to hard drives which can last over a decade without mechanical malfunction.
– Lower recoverability – After mechanical failure the data is completely lost as the cell is destroyed, while if normal HDD suffers mechanical failure the data is often recoverable using expert help.

출처: 위키백과

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