The Inevitable Rise of SSDs
So what about SSDs, or solid-state drives? They're by far the buzziest of the storage options, and we're constantly told that solid-state drives will replace hard drives, like, now. That's not quite right. Solid-state drives, which have no moving parts and store data with electrical charge rather than magnetism, are taking over—just, not everything.
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. Recently, NAND based flash memory has become the standard for most SSD's.
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 electromechanical 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.
Comparison of SSD with hard disk drives
A comparison of SSDs, Secure Digital High Capacity (SDHC) drives, and hard disk drives (HDDs). The disassembled components of a hard disk drive (left) and of the PCB and components of a solid-state drive (right) Comparisons reflect typical characteristics, and may not hold for a specific device.
Advantages
Faster start-up because no spin-up is required.
Fast random access because there is no read/write head
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.
Consistent read performance because physical location of data is irrelevant for SSDs.
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 SSDs have twice the data density of HDDs (so far, with very recent and major developments of improving SSD densities), even up to 1TB disks (currently more than 2TB is atypical even for HDDs). 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.
Defragmenting the SSD is unnecessary. Since SSDs are random access by nature and can perform parallel reads on multiple sections of the drive (as opposed to a HDD, which requires seek time for each fragment, assuming a single head assembly), a certain degree of fragmentation is actually better for reads, and wear leveling intrinsically induces fragmentation.[citation needed] In fact, defragmenting a SSD is harmful since it adds wear to the SSD for no benefit.
Disadvantages
Wear leveling used on flash-based SSDs has security implications. For example, encryption of existing unencrypted data on flash-based SSDs cannot be performed securely due to the fact that wear leveling causes new encrypted drive sectors to be written to a physical location different from their original location—data remains unencrypted in the original physical location. It is also impossible to securely wipe files by overwriting their content on flash-based SSDs.
As of early-2010, SSDs are still more expensive per gigabyte than hard drives. Whereas a normal flash drive is US$2 per gigabyte, hard drives are around US$0.10 per gigabyte for 3.5", or US$0.20 for 2.5".
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 drives have limited lifetimes and will often wear out after 1,000,000 to 2,000,000 write cycles (1,000 to 10,000 per cell) for MLC, and up to 5,000,000 write cycles (100,000 per cell) 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), 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.
Commercialization
Cost and capacity
Until recently, flash based solid-state drives were too costly for widespread use in mobile computing. As flash manufacturers transition from NOR flash to single-level cell (SLC) NAND flash and most recently to multi-level cell (MLC) NAND flash to maximize silicon die usage and reduce associated costs, "solid-state disks" are now being more accurately renamed "solid-state drives" – they have no disks but function as drives – for mobile computing in the enterprise and consumer electronics space. This technological trend is accompanied by an annual 50% decline in raw flash material costs, while capacities continue to double at the same rate. As a result, flash-based solid-state drives are becoming increasingly popular in markets such as notebook PCs and sub-notebooks for enterprises, Ultra-Mobile PCs (UMPC), and Tablet PCs for the healthcare and consumer electronics sectors. Major PC companies have now started to offer such technology. It has been said that using said flash drive incurs an overall speed increase of 250%.
Availability
Solid-state drive (SSD) technology has been marketed to the military and niche industrial markets since the mid-1990s.
CompactFlash card used as SSD
Along with the emerging enterprise market, SSDs have been appearing in ultra-mobile PCs and a few lightweight laptop systems, adding significantly to the price of the laptop, ending on the capacity, form factor and transfer speeds. As of 2008 some manufacturers have gun shipping affordable, fast, energy-efficient drives priced at $350 to computer manufacturers. For low-end applications, a USB flash drive may be obtained for $10 to $100 or so, depending on capacity, or a CompactFlash card may be paired with a CF-to-IDE or CF-to-SATA converter at a similar cost. Either of these requires that write-cycle endurance issues be managed, either by not storing frequently written files on the drive, or by using a Flash file system. Standard CompactFlash cards usually have write speeds of 7 to 15 megabytes per second while the more expensive upmarket cards claim speeds of up to 40 MB/s.
One of the first mainstream releases of SSD was the XO Laptop, built as part of the 'One Laptop Per Child' project. Mass production of these computers, built for children in developing countries, began in December 2007. These machines use 1024 MiB SLC NAND flash as primary storage which is considered more suitable for the harsher than normal conditions in which they are expected to be used. Dell began shipping ultra-portable laptops with SanDisk SSDs on April 26, 2007. Asus released the Eee PC subnotebook on October 16, 2007, and after a successful commercial start in 2007, it was expected to ship several million PCs in 2008, with 2, 4 or 8 gigabytes of flash memory. On January 31, 2008, Apple Inc. released the MacBook Air, a thin laptop with optional 64 GB SSD. The Apple store cost was $999 more for this option, as compared to that of an 80 GB 4200 rpm Hard Disk Drive. Another option—Lenovo ThinkPad X300 with a 64Gbyte SSD—was announced by Lenovo in February 2008, and is, as of 2008, available to consumers in some countries. On August 26, 2008, Lenovo released ThinkPad X301 with 128GB SSD option which adds approximately $200 US.
The Mtron SSD
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. In May 2009 Toshiba launched a laptop with a 512 GB SSD. In December 2009, Micron Technology announced the world's first SSD using a 6Gbps SATA interface.
Quality and performance
SSD is a rapidly 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 produces the best performing SSD drive as of 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, inadvertently 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 were really impressive. They all have significant weaknesses: usually either low I/O performance, poor write throughput or unacceptable power consumption".
OCZ has recently unveiled OCZ Vertex 2 Pro which is currently the fastest MLC SSD drive with a Sandforce Controller onboard performing more or less as the Intel X25-E series SSD drives.
