A flash memory card (sometimes called a storage card) is a small storage device that uses nonvolatile semiconductor memory to store data on portable or remote computing devices. Such data includes text, pictures, audio and video. Most current products use flash memory, although other memory technologies are being developed, including devices that combine dynamic random access memory (DRAM) with flash memory.
Types of memory card formats: Consumer devices
There are a number of flash memory card types on the market, roughly divided between consumer devices and enterprise storage devices.
These devices include the Secure Digital card (SD card) and its smaller variant, the microSD card; Secure Digital High Capacity (SDHC) card; CompactFlash card (CF card); SmartMedia card; Memory Stick; MultiMediaCard (MMC); xD-Picture card; and USB card.
The above types of memory cards are usually associated with consumer devices, such as digital cameras, smartphones and tablets. The cards come in varying sizes, and storage capacities typically correspond directly to their price.
Most types of memory cards available have constantly powered, nonvolatile memory, particularly NAND flash. Nonvolatile memory safeguards data in the event of a power outage, software bug or other disruption, and also eliminates the need to periodically refresh data on the memory card. Because memory cards use solid-state media, they involve no moving parts and are less likely to suffer mechanical difficulties.
Evolution of the flash memory card
Earlier removable storage media, such as the PC card, smart card and similar cards used for computer gaming systems, are also considered memory cards. However, the newer types of memory cards are smaller, require less power, have higher storage capacities and are portable among a greater number of devices. Because of these features, memory cards are influencing the production of an increasing number of small, lightweight and low-power devices.
Memory cards offer a number of advantages over a hard disk drive (HDD): they are much smaller and lighter, extremely portable, silent, allow more immediate access and are less prone to mechanical damage. However, an HDD still offers a compelling advantage: Although flash prices are coming down, a typical memory card still costs more (and has a lower storage capacity) than a high-capacity HDD.
An overview of consumer-oriented flash memory cards
Here are brief descriptions of the major consumer-oriented flash memory cards, including when they were introduced and their use cases.
SD card: Postage stamp-sized device widely used in a variety of handheld devices, including digital cameras, smartphones and tablets. SD cards secure digital data by encrypting it on the device. SanDisk offers the highest capacity SD card at 512 gigabytes (GB).
MicroSD: In 2005, SanDisk and Motorola teamed up to introduce the original microSD product, then known as TransFlash, as a 128 GB removable card for mobile phones. In June 2016, SanDisk (now part of Western Digital Corp.) launched a suite of 256 GB microSD cards, including Ultra microSDHC and microSDXC UHS-I cards geared for Android-based devices.
CompactFlash drive technology: The forerunner to the SD card, the original CF cards were designed on the Parallel Advanced Technology Attachment standard and were no larger than a matchbook. CF cards included a microcontroller and were used as flash memory storage for high-resolution photography. CF and SD cards lack built-in USB computer device connectivity.
MultiMediaCard: Developed in 1997 by SanDisk and Siemens, MMCs were originally designed to use NAND flash memory technology from Toshiba. However, MMCs are less common with the arrival of SD card technology. Most computer hardware vendors no longer provide ports for inserting an MMC device. A new development is embedded MMC, or eMMC, in which the flash card is integrated on the computer motherboard along with controller software to use the eMMC as a bootable system drive. MMCs weigh approximately two grams.
SDHC card: This card has the same form factor as an SD card, with specifications that define SDHC card capacities from 4 GB to 32 GB. These devices were developed to tackle high-definition video and high-resolution images. Although SD cards will work in an SDHC device, an SDHC card will not function in an SD card-based digital camera or card reader.
Memory stick: Sony developed its Memory Stick technology as a removable flash storage device to transfer photos and high-definition video.
USB drive: This portable plug-and-play flash storage device is inserted into a computer's standard USB port. USB drives ushered in the demise of floppy disks and, to some extent, the reduced use of compact discs.
XD-Picture card: A card format designed for use only with Olympus digital cameras.
Consumer solid-state drives flash cards
Consumer-grade solid-state drives (SSDs) embed silicon-based memory chips as storage media for persistent storage of data. The earliest SSDs were generally designed for consumer devices. The debut of the Apple iPod in 2005 marked the first notable flash-based device to broadly penetrate the consumer market. SanDisk has a line of portable SSDs that scale to 1.92 terabytes (TB) of flash capacity; they are marketed mainly for flash storage of digital photography.
The earliest commercially designed SSDs were made with single-level cell (SLC) or multi-level cell (MLC) flash. SLC uses a high grade of flash media to provide performance and endurance, but it typically costs twice as much as MLC flash.
Antony Adshead, a storage editor with Computer Weekly, explains the basics of flash storage technology and offers a rundown of MLC, eMLC, SLC and TLC flash, commenting on their place in the market and the use cases they are suited to.
Some consumer-grade flash memory cards are used by companies to cache reads and writes with hybrid flash storage. Enterprise MLC is an MLC NAND variant with enhanced write cycles compared to consumer-grade MLC. Some newer enterprise SSDs use triple-level cell NAND flash, which stores three data bits per each flash cell. SSDs made with 3D NAND represent the next evolution. IBM, Samsung and Toshiba produce and market SSDs with 3D NAND, in which flash memory cells are stacked atop one another in vertical layers.
In most respects, the above types of flash memory cards differ from those used in enterprise storage. EMC is credited with being the first vendor to include SSDs in enterprise storage hardware when it added them to its Symmetrix disk arrays in 2008, spawning the advent of hybrid arrays that combine flash drives with a traditional spinning disk. Initially, enterprise SSDs in hybrid arrays were relegated for caching read data in flash due to their higher cost and lower endurance compared to HDDs.
SSDs are commonly available in form factors similar to traditional HDDs: 1.8-inch, 2.5-inch and 3.5-inch. SSDs can be inserted in slots in computer servers -- referred to as Server-side flash storage -- or used as part of an enterprise flash storage array system.
PCIe-based SSDs are designed around Peripheral Component Interconnect Express, a high-speed expansion card format that connects a computer with its attached peripherals. PCIe has a point-to-point architecture, allowing each device to connect to a host via its own serial link, rather than by sharing a network bus. By virtue of this direct connection, PCIe SSDs are generally rated to deliver higher performance than server-based Serial Advance Technology Attachment (SATA), Serial-Attached SCSI (SAS) or Fibre Channel (FC) SSDs.
Flash memory cards in servers
Aside from SSDs, other form factors for server-based flash storage include:
- Add-in card SSDs that fit in a PCIe serial connection.
- Disk-on-module devices that mount to a motherboard as a flash-based boot drive.
- Flash dual inline memory module (DIMM), otherwise known as memory channel solid-state storage, which exploits idle DIMM DRAM slots as low-latency, high-performance flash cache to buffer host server performance.
- mSATA flash drives mainly used in laptops, netbooks and other portable computing devices. The mSATA specification maps SATA signals to an internally mounted PCIe card in a computer motherboard. The M.2 SSD form factor for ultrathin computing devices is generally considered to be the eventual replacement for mSATA-based flash memory cards.
- Nonvolatile DIMMs (NVDIMM) that combine DRAM with nonvolatile memory, usually NAND flash. There are three categories of NVDIMM cards.
- NVDIMM-N devices that make only the DRAM visible to the system. The flash component is not addressable storage and serves only as a backup to DRAM.
- NVDIMM-F, also known as memory channel flash, contains flash and no DRAM. NVDIMM-F capacities are similar to SSDs, with the earmark of lower latencies than standard flash storage.
- NVDIMM-P combines the functionality of NVDIMM-F and NVDIMM-N on one module. The nonvolatile memory is allocated in two ways. Part of it provides persistence to DRAM, while the remainder is available as block storage.
Emerging flash memory card specifications
The nonvolatile memory express (NVMe) protocol is a specification that permits an SSD to exploit the PCIe bus. NVMe operates at the host controller to define the command set and feature set needed to streamline the I/O stack. The aim is to enable PCIe-based SSDs to deliver low latency, higher throughput, and to consume less power when compared to SAS or SATA SSDs.
The nonprofit NVM Express Inc. is developing the NVMe over Fabrics industry specification to enable the NVMe storage interface to transfer data commands via Ethernet, FC, InfiniBand and other network fabrics.
Hybrid RAM-flash memory card technology
Micron Technology and Intel Corp. have previewed a form of storage-class memory known as 3D XPoint (pronounced "cross point"). The vendors said they will produce flash drives that store 128 GB across two stacked layers of flash memory. 3D XPoint reportedly would be approximately 1,000 times faster than NAND flash, 1,000 times more enduring than DRAM and provide 10 times the storage density of existing NAND flash-based SSDs when it hits the market.
Hewlett Packard Enterprise and SanDisk are partnering on a similar initiative to develop storage-class memory based on SanDisk's nonvolatile resistive random-access memory (ReRAM) technology, which would be used to design a new class of SSDs.