What is a SSD?


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.

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 the present context, has been adopted to distinguish solid-state electronics from electro-mechanical devices. 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 enjoy low access time and latency.

Advantages and Disadvantages of SSD


* Faster start-up because no spin-up is required.
* Fast random access because there is no read/write head
o Low read latency times for RAM drives. In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl’s law).
o Consistent read performance because physical location of data is irrelevant for SSDs.
o 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.This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions (flash).
* 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.
* Flash SSD’s have twice the data density of HDD’s (so far, with very recent and major developments of improving SSD densities), even up to 1TB disks[19][20] (currently more than 2TB is atypical even for HDD’s)). One example of this advantage is that portable devices such as a smartphone may hold as much as a typical person’s desktop PC.
* Failures occur less frequently while writing/erasing data, which means there is a lower chance of irrecoverable data damage.


* Even though technologies are developing at a fast pace, SSDs are still more expensive per gigabyte than hard drives. Whereas a normal flash drive is between US$2-3.45 per gigabyte, hard drives were around US$0.38 per gigabyte.
* 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.
* 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.
* Similarly, SSD write performance is significantly impacted by the availability of free, programmable blocks. Previously written data blocks that are no longer in use can be reclaimed by TRIM; however, even with TRIM, fewer free, programmable blocks translates into reduced performance.
* Flash-memory cells have limited lifetimes and will often wear out after 1,000,000 to 2,000,000 write cycles for MLC, and up to 5,000,000 write cycles for SLC. Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device, called wear leveling.
* As a result of wear leveling and write combining, the performance of SSDs degrades with use.
* SATA-based SSDs generally exhibit much slower write speeds. As erase blocks on flash-based SSDs generally are quite large (e.g. 0.5 – 1 megabyte),[8] they are far slower than conventional disks during small writes (write amplification effect) and can suffer from write fragmentation. Modern PCIe SSDs however have much faster write speeds than previously available.
* 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

Source: Wikipedia