A solid state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. An SSD emulates a conventional hard disk drive, thus easily replacing it in any application.

An SSD is commonly composed of either NAND flash (non-volatile) or SDRAM (volatile).

SSDs based on volatile memory such as SDRAM 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 drives, 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. (Though flash SSDs are significantly slower than DRAM SSDs).

Solid state drives are especially useful on a computer that has already come with maximum amount of RAM. For example, some x86 architectures with a 4 GB limit, can effectively be extended by putting the paging file or swap file on an SSD. These SSDs 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 a buffer cache mechanism. Whenever data is written to memory, the corresponding block in memory is marked as dirty and all dirty blocks can be flushed to the actual hard drive based on the following two strategies:
- Time (e.g. every 10 seconds, flush all dirty data),
- Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data).


Compared with hard disk drives (HDDs) :

Advantages
- Faster startup (as no spin-up is required).
- Faster random I/O (compared to hard disk drives).
- Extremely low read and write latency (seek) times, orders of magnitude faster than the best current hard disks drives.
- Faster boot and application launch time when hard disk seeks are the limiting factor. See Amdahl's law.
- In some cases, somewhat longer lifetime – Flash storage typically has a data lifetime on the order of 10 years before degradation. If data is periodically refreshed, it can store data indefinitely.
- Few to no moving parts.
- For small SSDs up to 64GB, lower power consumption and heat production.
- For small SSDs up to 64GB, no noise – Lack of moving parts makes the SSD completely silent (although high-end SSDs may include cooling fans).
- Better mechanical reliability – Lack of moving parts almost eliminates the risk of mechanical failure. High level of ability to endure extreme shock, high altitude, vibration and temperatures, which apply to laptops and other mobile devices, or when transported.
- Relatively deterministic performance – unlike hard disk drives, performance of SSDs is almost constant and deterministic across the entire storage. This is because "Seek time" can be constant, so fragmentation has less impact on performance than on physical drives.
- For very low-capacity SSDs, lower weight and size. Size and weight per unit storage are still better for traditional hard drives, and microdrives allow up to 20 GB storage in a CompactFlash 42.8×36.4×5 mm (1.7×1.4×.2 in) form factor. Up to 64GB, SSD is lighter than Hard drive for the same size

Flash based SSDs also have several disadvantages:
- Price – As of late 2007, flash memory prices are still considerably higher per gigabyte than those of comparable conventional hard drives – around US$15 per GB compared to typically less than US$1 for mechanical drives.[3]
- Today limited capacity, only 64GB actually and 128GB in may 2008. 128GB is already available
- Slow random write speeds – as erase blocks on SSDs generally are quite large, they're far slower than conventional disks for random writes.
- In some cases, SSDs have substantially lower throughput than conventional hard disks. In spite of the decreased latency, this can lead to dramatically lower performance than hard disk drives. More expensive SSDs can have much greater bandwidth than HDDs, so this isn't universally a problem.

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* Solid_state_drive