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A solid-state drive (SSD) is a data storage device that uses solid-state memory to store persistent data. An 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 in this context, has been adopted to distinguish solid-state electronics from electromechanical devices as well. With no moving parts, solid-state drives are less fragile than hard disks and are also silent (unless a cooling fan is used); as there are no mechanical delays, they usually employ low access time and latency.

SSDs have begun to appear in laptops,

Flash drives

Most SSD manufacturers use non-volatile flash memory to create more rugged and compact devices for the consumer market. These flash memory-based SSDs, also known as flash drives, do not require batteries. They are often packaged in standard disk drive form factors (1.8-inch, 2.5-inch, and 3.5-inch). In addition, non-volatility allows flash SSDs to retain memory even during sudden power outages, ensuring data persistence. SSDs are slower than DRAM and some designs are slower than even traditional HDDs on large files, but flash SSDs have no moving parts and thus seek times and other delays inherent in conventional electro-mechanical disks are negligible.

Components:

The performance of the SSD can scale with the number of parallel NAND flash chips used in the device. A single NAND chip is relatively slow, due to narrow (8/16 bit) asynchronous IO interface, and additional high latency of basic IO operations (typical for SLC NAND - ~25 μs to fetch a 4K page from the array to the IO buffer on a read, ~250 μs to commit a 4K page from the IO buffer to the array on a write, ~2 ms to erase a 256 KB block). When multiple NAND devices operate in parallel inside an SSD, the bandwidth scales, and the high latencies can be hidden, as long as enough outstanding operations are pending and the load is evenly distributed between devices.

Micron/Intel SSD made faster flash drives by implementing data striping (similar to RAID0) and interleaving. This allowed creation of ultra-fast SSDs with 250 MB/s effective read/write, the maximum the SATA interface can manage.

SLC versus MLC

Lower priced drives usually use multi-level cell (MLC) flash memory, which is slower and less reliable than single-level cell (SLC) flash memory. (RAM & flash).

Disadvantages

As of October 14, 2008, Apple's MacBook and MacBook Pro lines carry optional solid state hard drives of up to 256 GB at an additional cost. Dell began to offer optional 256 GB solid state drives on select notebook models in January 2009.

In late 2008, Sun released the ''Sun Storage 7000 Unified Storage Systems'' (codenamed Amber Road), which use both solid state drives and conventional hard drives to take advantage of the speed offered by SSDs and the economy and capacity offered by conventional hard disks.

Quality and performance

SSD is still currently a developing technology. A January 2009 review of the market by technology reviewer Tom's Hardware concluded that comparatively few of the tested devices showed acceptable I/O performance, with several disappointments, and that Intel (who make their own SSD chipset) still produce the best performing SSD drive as at this time, a view also echoed by Anandtech. In particular, operations that require many small writes, such as log files, are particularly badly affected on some devices, potentially causing the entire host system to freeze for periods of up to one second at a time.

According to Anandtech, this is due to controller chip design issues with a widely used set of components, and at least partly arises because most manufacturers are memory manufacturers only, rather than full microchip design and fabrication businesses - they often rebrand others' products, inadvertantly replicating their problems. Of the other manufacturers in the market, Memoright, Mtron, OCZ, Samsung and Soliware were also named positively for at least some areas of testing.

The overall conclusion by Tom's Hardware however, was that ''none of the [non-Intel] drives was really impressive. They all have significant weaknesses: usually either low I/O performance, poor write throughput or unacceptable power consumption''. Microsoft created ReadyBoost to exploit characteristics of USB-connected flash devices, but Windows operating systems are not optimized for use of SSDs. Microsoft's latest OS Windows 7 which is now in RC, is optimized for SSDs as well as hard disks.

SSDs and [[ZFS

Solaris, as of 10u6 (released in October 2008), and recent versions of OpenSolaris and Solaris Express Community Edition on which OpenSolaris is based, can use SSD drives as a performance booster for ZFS. There are two available modes -- using an SSD for the ZFS Intent Log (ZIL), which is used every time a write to the disk occurs, or for the L2ARC (Level 2 Adaptive Replacement Cache), which is used to cache data for reading. When used either alone or in combination, large increases in performance are generally seen.