In computers, a storage medium is any technology -- including devices and materials -- used to place, keep and retrieve electronic data. It refers to a physical device or component in a computing system that receives and retains information relating to applications and users. The plural form of this term is storage media.
Early forms of storage media included computer paper tape. Holes punched in the paper corresponded to a single bit of data. A paper tape reader would interpret each punched hole and convert it to a number. Paper tape was supplanted by magnetic tape, which eventually evolved to magnetic floppy disk.
How storage media works
Media used in computer storage receive messages in the form of data, via software commands from the computer system. The commands determine the type of storage media needed to hold the data, based on its business value, compliance implications or other factors. In tiered storage, data is moved among disk, flash and cloud storage based on automated software policies.
A storage medium may be internal to a computing device, such as a computer's hard drive, or a removable device such as an external hard drive or universal serial bus (USB) flash drive. There are various types of storage media, including magnetic tape, nonvolatile memory cards, rotating fixed disk and solid-state drives (SSDs), which are based on nonvolatile flash memory.
The term storage encompasses all data, and can be either primary or secondary storage. Primary storage refers to data that is kept in memory for fast retrieval by a computer's processor. Secondary storage is data placed on hard disk or tape to ensure backup and long-term retention.
A storage device may be a type of storage media, or a piece of storage hardware outfitted with storage media. For example, storage arrays decouple storage media from servers. Storage arrays incorporate electromechanical hard disk drives (HDDs), SSDs or a combination of each, attached to separate servers and networking.
Storage media can be arranged for access in many ways. Some well-known arrangements include:
- redundant array of independent disks (RAID);
- network-attached storage (NAS); and
- storage area network (SAN).
SAN arrays initially were designed with HDDs, until the advent of all-flash arrays outfitted solely with SSDs. Hybrid flash arrays blend the two storage media in an integrated system, with disk providing a capacity tier alongside a faster tier of flash.
Examples of storage media, and the pros and cons of each, are as follows.
A hard disk provides a high-capacity alternative to magnetic storage media. It contains metal platters coated with a magnetic layer. The platters usually spin continuously when a computer is on, storing data in different sectors on the magnetic disk.
Magnetic disk remains the dominant media for backup storage appliances, active archives and long-term retention. A disk-based backup appliance includes interfaces to replicate data copies, such as clones and snapshots, to tertiary devices or a hybrid cloud.
HDDs remain popular in enterprise disk arrays due to their increasing capacities and the ability to rewrite data on the disk. In 2017, Western Digital Corp. introduced a 14 TB HDD, making it the largest on the market at that time. Seagate Technology has said it plans to unveil a 16 TB HDD in 2018. A downside to HDDs is the reliance on moving internal mechanisms such as actuators, motors and spindles that can fail and corrupt the drive.
Some HDDs use shingled magnetic recording (SMR) as an alternative to conventional magnetic recording. An SMR method allows for greater areal density by allowing data to be written in partially overlapping tracks on the disk. SMR drives work optimally with data that is continuously written, such as with disk-based archiving and backup. Seagate and Western Digital are among the leading manufacturers of SMR-based disks.
RAID works by placing data on multiple disks and allowing input/output (I/O) operations to overlap in a balanced way, improving performance. In the event a drive fails, the data is protected from companion drives.
Because the use of multiple disks increases the mean time between failures (MTBF), storing data redundantly also increases fault tolerance.
Optical disk technology uses lasers for write once, read many (WORM) data. The use of lasers allows high-density optical disk to store more data than magnetic HDDs. Types of optical storage media include Blu-ray, DVDs and CD-ROMs for read-only data.
Flash memory does not depend on moving mechanical parts. This gives flash devices advantages in speed over traditional disks. In flash memory, blocks of data must be erased to allow new data to be written to the microchip.
The two main types of flash are NAND and NOR. The names are defined by their respective logic gates. These memory types are used as the storage media in SSDs.
NAND flash memory is written and read in blocks that are smaller than the device, whereas NOR flash memory reads and writes bytes independently. Both types of flash are used in consumer electronics and certain applications in medical and industrial use cases. NAND flash is known for higher density and higher endurance than NOR, making it suitable for enterprises to use in mass storage. NOR often serves as a replacement for random access memory (RAM) and read-only memory (ROM) drives.
Single-level cell NAND stores 1 bit of data per flash cell, existing as either a programmed (0) or erase (1) state. Multi-level cell (MLC) NAND stores more than 1 bit of data per flash cell. Enterprise MLC is made with enhancements to support more write cycles than consumer-grade MLC. Triple-level cell stores three data bits per cell.
Flash storage memory devices are divided between consumer devices and enterprise storage, with some overlap. Smartphones, tablets and other consumer devices are designed memory cards that vary in capacity and price.
An SSD is installed in x86 computers to allow companies to user server-side flash as an alternative or adjunct to networked storage arrays. Form factors include:
- Add-in cards that utilize a Peripheral Component Interconnect Express (PCIe) serial port.
- Disk-on-module flash boot drives that mount to a computer's motherboard.
- Dual inline memory modules (DIMMs) place flash close to the motherboard in dynamic random-access memory (DRAM) slots as a performance cache.
- MiniSATA drives and their eventual replacement m.2 SSDs, used in thin laptops.
- Storage-class memory, including nonvolatile DIMMs with DRAM as addressable storage and flash as backup media. 3D XPoint technology -- developed by Intel and Micron -- is an example of storage-class memory.
SSDs initially were designed to take advantage of existing Serial-Attached SCSI (SAS) and Serial-Advanced Technology Attachment (SATA) protocols.
Development of the nonvolatile memory express (NVMe) protocol exploits idle PCIe ports in a computer to allow an application to communicate directly with a data storage device. PCIe-based NVMe SSDs aim to reduce latency and boost throughput. Most major SSDs support PCIe, SAS and SATA. NVMe over Fabrics represents the next expected phase, enabling NVMe commands to transfer data between a host and flash storage device using an Ethernet, Fibre Channel or InfiniBand connection.
USB flash drives
USB flash drives are also known as nearline storage, a storage medium that is not continuously connected to network servers or the internet. Generally, this makes most removable media, such as encrypted cartridges or SATA drives, safe from infection by Trojan horses, viruses or worms.
A USB flash drive specifically refers to the housing for the device; the storage media is internal flash designed as an integrated memory circuit, similar to the design of SSDs but on a smaller scale. These thumb-shaped devices slide into any USB to transfer or copy data, and are variously called gum sticks, keychain drives and jump drives.
Although USB drives are widely used by consumers due to their convenience, that ease of use can present a security hazard within enterprises. Most companies forbid employees to use personal USB drives at work for security reasons, unless specifically authorized to do so.
Other removable flash storage media include the Secure Digital card/microSD card, Secure Digital High Capacity card, CompactFlash card, SmartMedia card, Sony Memory Stick, MultiMediaCard and the xD-Picture card, all of which are found mainly in consumer electronics.
Tape was a dominant backup storage medium until the 1990s but was gradually pushed aside by magnetic disk. Tape systems remain in use, but the use case now centers on high-capacity archiving for preserving data. Tape systems have continued to improve in density and endurance, largely due to advances in the Linear Tape-Open (LTO) format. LTO-8 pushes compressed capacity per tape to 32 TB and nearly 13 TB of uncompressed data.
Tape libraries are composed of hundreds and hundreds of physical tapes, presented in a system that allows users to add or remove tapes, track a tape's location and set mount points for accessing the data on tape.
An offshoot is the rise of virtual tape library systems for backup, in which an array of physical disks gets presented as tape to the backup software. Data is written sequentially as if to tape, but reads and writes occur faster since they are retrieved from magnetic disk.