Definitions

Ram drive

Solid-state drive

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

SSDs have begun to appear in laptops, although they are at present substantially more expensive per unit of capacity than hard drives.

History

The first ferrite memory SSD devices, or auxiliary memory units as they were called at the time, emerged during the era of vacuum tube computers. But with the introduction of cheaper drum storage units, their use was discontinued. Later, in the 1970s and 1980s, SSDs were implemented in semiconductor memory for early supercomputers of IBM, Amdahl and Cray; however, the prohibitively high price of the built-to-order SSDs made them quite seldom used product.

In 1978 StorageTek developed the first modern type of solid-state drive. In the mid-1980s Santa Clara Systems introduced BatRam, an array of 1 megabit DIP RAM Chips and a custom controller card that emulated a hard disk. The package included a rechargeable battery to preserve the memory chip contents when the array was not powered. The Sharp PC-5000, introduced in 1983, used 128 kilobyte (128 KiB) solid-state storage cartridges, containing bubble memory.

RAM "disks" were popular as boot media in the 1980s when hard drives were expensive, floppy drives were slow, and a few systems, such as the Amiga series, the Apple IIgs, and later the Macintosh Portable, supported such booting. At the cost of some main memory, the system could be soft-rebooted and be back in the operating system in mere seconds instead of minutes. Some systems were battery-backed so contents could persist when the system was shut down.

In 1995 M-Systems introduced flash-based solid-state drives. (SanDisk acquired M-Systems in November 2006). Since then, SSDs have been used successfully as hard disk drive replacements by the military and aerospace industries, as well as other mission-critical applications. These applications require the exceptional mean time between failures (MTBF) rates that solid-state drives achieve, by virtue of their ability to withstand extreme shock, vibration and temperature ranges.

In 2007, SSD's of a few gigabytes capacity gained mainstream popularity with netbook and subnotebooks.

Enterprise Flash Drives (EFDs) are designed for applications requiring high performance ( Input/Output Operations Per Second), reliability and energy efficiency.

Architecture and function

An SSD is commonly composed of either DRAM volatile memory or NAND flash non-volatile memory.

Flash based

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. Up to recently flash SSDs were significantly slower than DRAM (and even traditional HDDs on big files), but still perform better than traditional hard drives (at least with regard to reads) because of negligible seek time (flash SSDs have no moving parts, and thus eliminate spin-up time, and greatly reduce seek time, latency, and other delays inherent in conventional electro-mechanical disks).

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 read/write - the maximum SATA interface could manage. This makes RAM-based SSDs nearly obsolete.

SLC versus MLC

Lower priced drives usually use Multi-level cell (MLC) flash memory, which is slower and somewhat less reliable than Single-level cell (SLC) flash memory. Still, MLC chips continue to be used in many commericially available SSD's.

DRAM based

SSDs based on volatile memory such as DRAM are characterized by ultra fast 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 SDDs or traditional HDDs. DRAM-based SSDs usually incorporate internal battery 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 data is copied from random access memory (RAM) to back-up storage, or to allow the data's transfer to another computer. When the power is restored, the data are copied back to RAM from the back-up storage, and the SSD resumes normal operation. (Similar to the hibernate function used in modern operating systems.)

These types of SSD are usually fitted with the same type of DRAM modules used in regular PC's and servers, allowing them to be swapped out and replaced with larger modules.

A secondary computer with a fast network connection can be used as a RAM-based SSD.

DRAM based solid-state drives are especially useful on computers that already have the maximum amount of supported RAM. For example, some computer systems built on the x86-32 architecture can effectively be extended beyond the 4 GB limit by putting the paging file or swap file on an SSD. Owing to the bandwidth bottleneck of the bus they connect to, DRAM SSDs cannot read and write data as fast as main RAM can, but they are far faster than any mechanical hard drive. Placing the swap/scratch files on a RAM SSD, as opposed to a traditional hard drive, therefore can increase performance significantly.

Gigabyte i-RAM

The Gigabyte i-RAM (or GC-RAMDISK) is the main consumerised DRAM SSD system and uses standard DDR modules and connects to its host via Serial ATA. This card can use the system's standby power (also used for Wake-on-LAN and similar features) to maintain its RAM contents even with the system powered off, and includes a battery that can retain the data when the system is completely disconnected from power. Early versions were in the form of a PCI card but the PCI interface was only used to supply power to the card. Later versions used the 5.25" drive format.

Comparison with hard disk drives

A comparison (with benchmarks) of SSDs, Secure Digital High Capacity (SDHC) drives, and hard disk drives (HDDs) is given in the reference.

Advantages

  • Faster start-up, as no spin-up is required. (RAM & Flash)
  • Typically, fast random access for reading, as there is no read/write head to move. (RAM & Flash)
  • Extremely low read latency times, as SSD seek-times are orders of magnitude lower than the best current hard disk drives. (RAM) In applications where hard disk seeks are the limiting factor, this results in faster boot and application launch times (see Amdahl's law). (RAM)
  • Extremely fast write (RAM only)
  • No noise: a lack of moving parts makes SSDs completely silent, unless, as in the case of some high-end and high-capacity models, they have cooling fans attached. (RAM & Flash)
  • For low-capacity flash SSDs, low power consumption and heat production when in active use, although high-end SSDs and DRAM-based SSDs may have significantly higher power requirements. (Flash)
  • High mechanical reliability, as the lack of moving parts almost eliminates the risk of mechanical failure. (RAM & Flash)
    • Ability to endure extreme shock, high altitude, vibration and extremes of temperature: once again because there are no moving parts. This makes SSDs useful for laptops, mobile computers, and devices that operate in extreme conditions. (Flash)
  • Larger range of operating temperatures. Typical hard drives have an operating range of 5-55 degrees C. Most flash drives can operate at 70 degrees, and some industrial grade drives can operate over an even larger temperature range.
  • Relatively deterministic read performance: unlike hard disk drives, performance of SSDs is almost constant and deterministic across the entire storage. This is because the seek time is almost constant and does not depend on the physical location of the data, and so, file fragmentation has almost no impact on read performance.
  • 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 42.8×36.4×5 mm (1.7×1.4×.2 in) form-factor. Up to 256 GB, SSDs are currently lighter than hard drives of the same capacity.

Disadvantages

  • Price – as of mid-2008, SSD prices are still considerably more costly per gigabyte than are comparable conventional hard drives: consumer grade drives are typically USD 2 to 3.50 per GB for flash drives and over USD 80 per GB for RAM-based compared to less than USD 0.15 per gigabyte for hard drives.
  • Capacity – currently far lower than that of conventional hard drives. (Flash SSD capacity is predicted to increase rapidly, with experimental drives of 1 TB)
  • DRAM based SSDs have a higher vulnerability to abrupt power loss.
  • Limited write (erase) cycles – flash-memory cells will often wear out after 1,000 to 10,000 write cycles for MLC, and up to 100,000 write cycles for SLC, while high endurance cells may have an endurance of 1–5 million write cycles (many log files, file allocation tables, and other commonly used parts of the file system exceed this over the lifetime of a computer). Special file systems or firmware designs can mitigate this problem by spreading writes over the entire device (so-called wear levelling), rather than rewriting files in place. In 2008 wear levelling was just beginning to be incorporated into consumer level devices. An example for the lifetime of SSD is explained in detail in this wiki SSDs based on DRAM, however, do not suffer from this problem.
  • 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 for random writes and therefore vulnerable to write fragmentation, and in some cases for sequential writes. SSDs based on DRAM do not suffer from this problem.
  • Lower storage density – hard disks can store more data per unit volume than DRAM or flash SSDs, except for very low capacity/small devices.
  • Higher power consumption at idle or under low workloads laptop battery runtimes decrease when using an SSD over a 7200 RPM 2.5" laptop hard drive, flash drives also take more power per gigabyte.
    • RAM based SSD require more power than hard disks, both operating and when turned off.

Commercialization

Cost and capacity

Until recently, solid-state drives were too costly for 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.

Availability

Even though solid-state drive (SSD) technology has been marketed to the military and niche industrial markets since the mid-1990s, it is only recently that the enterprise sector has taken notice of the benefits that SSDs can offer, as key SSD technologies emerge, prices drop and new case studies, along with analyst reports, are published.

Along with the emerging enterprise market, SSDs have been appearing in ultra-mobile PCs and a few lightweight laptop systems, adding a US$ $600 to $1000 premium to the price of a HDD-equipped laptop, depending on the capacity, form factor and transfer speeds. Only a handful of companies offer large (128 GB or larger) SSD drives with write speeds adequate for replacing traditional drives, and these drives are available in limited quantities and are very expensive. Already some manufacturers have begun shipping affordable, fast, energy-efficient drives priced at $350 to computer manufacturers. For low-end applications, a USB memory stick may be used as a Flash hard drive 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 under the 'One Laptop Per Child' project. Mass production of these computers built for children in developing countries begun in December 2007. These machines use 1024 MiB SLC NAND flash as primary storage solution which is considered more suitable for the harsher than normal conditions they are expected to be used in. Dell has begun 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, expects 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 cost is $999 more for this option if configured in the Apple Store, 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 currently available to consumers in some countries.

Applications

A use for flash drives is to run lightweight operating systems designed specifically for turning general-purpose PCs into network appliances comparable to more expensive routers and firewalls. In this situation, a write protected flash drive containing the whole operating system is used to boot the system. A similar system could boot from CD, floppy disk or a traditional hard drive but flash memory is a good choice because of very low power consumption and failure rate.

Hybrid drive

A hybrid disk uses a small SSD as a buffer for a larger drive.

DRAM-based SSDs may also work as a buffer cache mechanism (see Hybrid RAM drive). When data are 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 criteria:

  1. Time (e.g., every 10 seconds, flush all dirty data);
  2. Threshold (when the ratio of dirty data to SSD size exceeds some predetermined value, flush the dirty data).
  3. Loss of power/computer shutdown

Problems with SSDs on Windows

Many Windows users who purchase MLC disks are baffled by the slow performance of their flash drive . Windows is optimized for hard disk storage, handling data in small chunks (e.g. 0.5KiB); flash drives use larger page sizes (e.g. 4KiB). The factors that reduce the speed include the fact that Windows uses larger files, as well as the fact that Windows' background services constantly access the disk.

See also

References

External links

History

Technology Adoption

Performance Comparison

Misc Products

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