Solid-state drive (SSD)

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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, not to be confused with a RAM disk.

DRAM based drive

See also: I-RAM and Hyperdrive (storage)

SSDs based on volatile memory such as DRAM are characterized by ultrafast data access, generally less than 0.01 milliseconds, and are used primarily to accelerate applications that would otherwise be held back by the latency of Flash SSDs or traditional HDDs. DRAM-based SSDs usually incorporate either an internal battery or an external AC/DC adapter and backup storage systems to ensure data persistence while no power is being supplied to the drive from external sources. If power is lost, the battery provides power while all information is copied from random access memory (RAM) to back-up storage. When the power is restored, the information is copied back to the RAM from the back-up storage, and the SSD resumes normal operation. (Similar to the hibernate function used in modern operating systems.)

Advantages

• Faster start-up because no spin-up is required.
• Fast random access because there is no read/write head^[22]
  • Low read latency times for RAM drives.^[23] In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl's law).^[24].
  • Consistent read performance because physical location of data is irrelevant for SSDs.^[25]
  • File fragmentation has negligible effect.
• Silent operation due to the lack of moving parts.
• Low capacity flash SSDs have a low power consumption and generate little heat when in use.
• High mechanical reliability, as the lack of moving parts almost eliminates the risk of "mechanical" failure.
• Ability to endure extreme shock, high altitude, vibration and extremes of temperature.^[26][27] This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions (flash).^[24]
• For low-capacity SSDs, lower weight and size: although size and weight per unit storage are still better for traditional hard drives, and microdrives allow up to 20 GB storage in a CompactFlash form-factor. As of 2008 SSDs up to 256 GB are lighter than hard drives of the same capacity.^[26]
• Failures occur less frequently whilst writing/erasing data, which means there is a lower chance of irrecoverable data damage.^[28]

Disadvantages

• As of mid-2008, SSDs are still more expensive per gigabyte than hard drives. Whereas a normal flash drive is between US$1.50-3.45 per gigabyte, hard drives are around US$0.38 per GB.^[18][29]
• The capacity of SSDs is currently lower than that of hard drives. However, flash SSD capacity is predicted to increase rapidly, with drives of 1 TB already released for enterprise and industrial applications.^{[30][31][32][33][34]}
• Asymmetric read vs. write performance can cause problems with certain functions where the read and write operations are expected to be completed in a similar timeframe. SSDs currently have a much slower write performance compared to their read performance.^[35]
• Due to the low storage density of SSDs, hard disks can store more data per unit volume than DRAM or flash SSDs, except for very low capacity/small devices.
• Flash-memory cells have limited lifetimes and will often wear out after 1,000 to 10,000 write cycles for MLC, and up to 100,000 write cycles for SLC.^{[18][36][37][38]} Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device, called wear leveling.^[39]
• As a result of wear leveling and write combining, the performance of SSDs degrades with use ^[40][41].
• SSDs exhibit slow write speeds. As erase blocks on flash-based SSDs generally are quite large (e.g. 0.5 - 1 megabyte)^[18], they are far slower than conventional disks during small writes (write amplification effect) and can suffer from write fragmentation.^[42]
• DRAM-based SSDs (but not flash-based SSDs) require more power than hard disks, when operating; they still use power when the computer is turned off, while hard disks do not.^[43]

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