What is flash storage? - Definition from WhatIs.com
Part of the Storage hardware glossary:

Flash storage is any type of data repository or storage system that uses flash memory.

Flash memory is ubiquitous in small computing devices and becoming more common for larger applications. The size and complexity of flash-based systems varies for storage in wearable computing devices, embedded systems, smartphones, portable USB drives and more, all the way up to enterprise-class all-flash arrays. Flash is packaged in a variety of formats for different storage purposes.

Flash storage uses and benefits:

Flash memory has become more widely-used than the mechanical hard drive, even though it has not replaced it as the prevalent main storage in desktops.  In notebooks, however, flash storage offers the additional boon of being more resistant to the high-g (gravitational acceleration) bumps and drops the devices often receive in their mobile lives. This rugged nature allows the drives to maintain function through these events, which saves data. Flash is more prevalent in notebooks than desktops, and smartphones and MP3 players have pretty much abandoned the mechanical hard drive altogether. Flash easily beats it for both compactness and power consumption. Flash also remains the standard form of storage in digital cameras, tablets and digital camcorders. Photolithographic shrinks continue to enable increase capacity, which makes flash suitable for increasingly miniaturized applications.

How does flash storage work?

Flash storage’s memory is actually a form of EEPROM (electrically-erasable programmable read-only memory). Unlike standard EEPROM, however, flash is a non-volatile memory type. This means that it does not require power to maintain stored data integrity, so a system can be turned off – or lose power -- without losing data. Flash also erases whole blocks of data at a time rather than on a bit-by-bit level as conventional EEPROM does and so does not require complete erasure for a rewrite.

Flash is solid state storage, storing data using electricity in surface-mounted chips on a printed circuit board (PCB). There are no moving mechanical parts involved, which reduces power consumption. A typical SATA flash drive consumes 50 percent or less the power required by mechanical drives and may be capable of sequential read speeds more than 500MB/s in consumer drives -- faster than even the fastest enterprise-class mechanical hard drives. That is only a part of the picture, because access times are where flash really shines. Operating more like RAM than ROM, flash drives have no mechanical limitation for file access, which enables nanosecond seek times rather than the milliseconds required by mechanical hard drives -- several orders of magnitude less latency.

Most flash storage systems are composed of a memory chip and an access flash controller. The memory chip is used to store data; the controller manages access to the storage space on the memory unit. The flash controller is often multi-channel, working with a RAM cache that uses only 10 percent of the total drive capacity. The cache buffers the data going to and from a number of chips. Buffering enhances speed by reading and writing to the chips in parallel. 

The image below displays the inside of a USB flash drive. On the left is the flash memory chip; the controller is on the right.

Inside flash drive

The history of flash storage:

Both NOR and NAND flash, the two main types of flash memory, were invented in 1984 by Dr. Fujio Masuoka while he was working at Toshiba. In comparison to the slow process used by EEPROM, the new format’s ability to be programmed and erased in large blocks reminded Dr. Masuoka of a camera flash. NOR and NAND are named for the technology of the floating gates of the memory cells that hold data.

Intel’s interest was piqued by the fact that NOR enabled random access. The company released the first flash storage device in 1988. Toshiba followed with the first NAND flash storage in 1989.

See the Computer History Museum’s video about the history of flash memory. (Definition continues below)

Flash storage formats:

NOR offers memory addressing on a byte scale, enabling true random access, along with good read speeds.  It was this addressability that interested Intel in NOR, which the company often uses for its extensible firmware interface (EFI). NOR is more expensive per gigabyte (GB) than NAND because of its larger individual cell size. NOR also has slower write and erase times and is less durable than NAND when it comes to repeat reads, writes and especially erasures, where quantum tunneling of electrons is used to pierce the dielectric insulating material of the cell wall, which degrades the material over time. These characteristics make it a great replacement for EEPROM- or ROM-based firmware BIOS and EFI chips where the addressability and read speed is a boon while the rewrite durability is less of a concern. An OS, file storage or back up drive, on the other hand, might be more liable to expose the limitations of NOR in the resulting dead drives.

NAND offers greater write speeds and durability along with lower cost per GB.  The lower cost is partially a result of the NAND memory cell’s gate construction which is thinner, saving die space and reducing the overall size of a chip per GB.  NAND can come in single-level cell (SLC) and multi -level cell (MLC) forms, which include enterprise MLC (eMLC), TLC. SLC stores a single bit of information per cell. SLC generally offers greater speeds, especially when it comes to writes, greater longevity and fewer bit errors.  MLC provides storage capacity for more data, as its cell is capable of more levels of charge (or states), which allow it to store multiple bits of storage per cell. First generation MLC doubles capacity over SLC, TLC provides a third bit. The extra levels of charge along with smarter flash controllers and firmware can allow for bit error correction as well. Samsung especially has been hard at work trying to improve MLC forms of NAND flash and has brought flash into the terabyte range and created higher speed TLC.

Solid-state storage comparison

Name

Description

Useful for

Conductive metal-oxide (CMOx)

nonvolatile storage medium in which oxygen ions migrate between conductive and insulating metal-oxide layers within a single chip.

Emerging technology

Enterprise multi-level-cell (eMLC) flash

A form of multi-level-cell (MLC) flash that offers an increased number of program/erase (PE) cycles for extended life and reliability.

Data storage for medium and large business high performance computing.

Flash-based solid-state storage

Any data repository or system that usesflash memory. The size and complexity of such systems ranges from USB drives to enterprise-class array-based memory systems.

Data storage for a wide variety of users and environments where performance is crucial.

Magnetoresistive random-access memory (MRAM)

A method of storing data bits using magnetic charges instead of the electrical charges used by dynamic RAM (DRAM).

High-density solid-state storage; emerging technology.

Multi-level-cell (MLC) flash

An approach to flash memory in which two data segments can be written to the same cell, thereby doubling the storage capacity of single-level cell (SLC) flash.

Used in standalone, hybrid and all-flash storage systems, spanning personal, small business and enterprise computing.

NAND flash memory

Flash memory technology or devices constructed using NAND logic gates.

High speed storage for all types of devices, including those for consumers (personal electronics), small businesses and enterprises.

NOR flash memory

Low-density, random-access flash memory technology or devices constructed using NOR logic gates.

Typically used in mobile phones and personal electronics devices to store executable code.

Phase-change memory (PCM)

A form of computer RAM (random-access memory) that stores data by altering the state of the matter rapidly back and forth between amorphous and crystalline on a microscopic scale.

Emerging technology noted for exceptional switching speed and high storage density.

RAM-based  solid-state storage

A volatile solid-state storage media based on RAM technology that is relatively insensitive to the number of PE cycles.

High-speed computer memory applications for personal, business, and government environments.

Resistive RAM (RRAM)

A form of nonvolatile storage that operates by changing the resistance of a specially formulated solid dielectric material.

Emerging technology noted for exceptional switching speed and high storage density.

Single-level-cell (SLC) flash

A nonvolatile solid-state storage device or technology that provides enhanced reliability and performance relative to MLC and eMLC flash media.

High-speed data storage for medium and large businesses and government agencies.

 

Flash storage interfaces:

Flash storage for computer memory comes in a variety of interfaces including USB, SATA, M.2, and PCIe. With USB 3, external drives can enable speeds that internal hard drives just barely hit, making them a great candidate for portable OS drives. SATA 3Gb and 6Gb are the most common formats in desktops and notebook computers. At speeds well beyond 500MB/s a second read and write, the 6Gb version mostly eliminates bandwidth bottlenecks. PCIe and M.2 connected flash storage hold bandwidth that allows for future expansion and represents the extreme end of speed-demanding solutions, at 985MB/s and 1969MB/s respectively.

Flash in the data center:

Data center managers looking for ways to address the energy drain represented by hard drives are examining flash storage as a way to achieve green computing or green data center benchmarks. Businesses with I/O-intensive applications, such as credit card processing systems, have also found flash storage to be efficient and cost-effective. As a result, enterprise storage providers like EMC, chip makers like Samsung and server manufacturers like Sun Microsystems have all entered the flash storage market.

Flash for hobbyists:

Many enthusiasts have adopted flash storage. These users often have their OS, a few games and data-intensive applications such as audio-, video- and image-editing software on a flash drive or even a flash RAID array. Main storage of files that don’t require speed -- and also perhaps those subject to a lot of erasing -- could be stored on a cost-effective and potentially massive conventional hard drive. Other enthusiasts may use all flash, depending on whether they are enthusiastic about flash itself or its attributes like silence, speed and the elimination of bottlenecks. Flash has those bases covered.

As interest in flash storage has grown in the market, industry watchers have also pointed out a frequently overlooked caveat with flash: While its speed and random read access is far superior to traditional hard drives, in heavy use (especially for some forms), longevity is reduced. This reduction is due to flash's relatively limited tolerance for write-erase cycles. Taking that into consideration, along with the additional cost per GB and the particular workload they are designed for, users must carefully select and provision data for this medium. Manufactures are working on features like wear leveling and DRAM/NVRAM caching to provide flash storage with better performance while reducing flash SSD write amplification to improve reliability.

 

This was last updated in November 2014
Contributor(s): Matthew Haughn
Posted by: Margaret Rouse

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