What is Primary Storage? - Definition from WhatIs.com

Primary storage is the collective methods and technologies used to capture and retain digital information that is in active use and critical for an organization’s operations. Primary storage data is frequently accessed by applications or other hardware systems and business users.

In contrast, secondary storage is used for data that is less frequently accessed, or no longer accessed at all. Examples of secondary storage include devices or systems use to store backup data ( a second copy of current primary data) and archival data which is rarely if ever, accessed. Since secondary data has limited immediate usefulness and is infrequently accessed, it is often referred to as data at rest. Primary and secondary storage are components of a tiered-storage architecture which comprises at least the two primary and secondary levels of storage, although some environments have additional storage tiers. Regardless of the number of tiers storage, the primary tier is typically reserved for transactional data or mission-critical application data that requires high performance.

Types of Primary Storage

Depending on the computing environment, primary storage might consist of hard disks or flash-based solid-state drives installed locally on an application server or file server. Alternatively, the primary storage tier might be a centralized and shared storage-area network (SAN) or network-attached storage (NAS) array. Its how the storage resource is used rather than the type of storage architecture that determines it is primary storage or some other tier of data storage.

If an organization’s primary storage is used for high-transaction, random access applications such as database management systems, the tier will often be a SAN device (or set of devices). SAN systems are block oriented, which means they store and access data in chunks of a predetermined size. Block access operates similarly to the way the storage media—hard disk drives or solid-state drives—stores, catalogs and accesses the data it hosts, so it typically provides the high performance that database systems require.

A NAS array may also be used as primary storage, but it uses a different method of storing and accessing data than a SAN. NAS storage is also often referred to as file storage, as it overlays the storage media system with a directory and metadata that more closely aligns with how applications store their related data. Applications such as Microsoft Word or Excel find their documents or spreadsheets via the NAS directory which collects that various pieces of a file that may be scattered across a drive or multiple drives and presents the file to the application as a single unit. Because of this added layer, file-based systems like NAS tend to be slower than SANs, but if a NAS meets an organization’s requirements for primary storage, it can certainly function at that level.

Cloud-based storage may also serve as primary storage, but because it is remote, the circumstances under which it would be used as a primary storage resource are very different from those related to SAN or NAS. If an application is locally hosted in an organization’s data center, it will have to access its stored in a cloud service via a private or public network such as the Internet. The relative slowness of telecommunications-based data, which would yield performance and responsiveness that wouldn’t meet primary storage standards. However, if the application also resided in the same cloud system as the data, then performance might be adequate.

Types of Storage Media Used for Primary Storage

A primary storage system, whether it’s installed locally in a server, is networked and shared like a SAN or NAS, or is cloud-based, may be composed of various types of media. Hard-disk media is still the most prevalent type of data storage, but because high performance is typically required of primary storage, solid-state drives (SSDs) are increasingly replaced hard disks. In some cases, primary storage systems may employ both types of storage media, using the solid-state devices for fast access and processing of data and hard-disk drives for mass storage of the required data.

A more recent development in storage technology, storage-class memory, may also be used for primary storage if performance demands are particularly rigorous. Storage-class memory combines some of the features of solid-state storage with those of random-access memory (RAM) which works closely with a server’s CPU to feed data to an application as rapidly as possible. Unlike RAM, storage-class memory is persistent, which means it retains data even when power is cut off. And while storage-class memory is somewhat slower than RAM, it’s much faster than standard solid-state drives, which makes it ideal for use as primary storage for applications such as in-memory databases. It’s also considerably more expensive than conventional solid-state storage.

Legacy References of Primary Storage

Back when mainframe computers ruled the data center, the term primary storage often referred to the volatile memory in the computer, much like RAM in modern servers, rather than the media that provided a permanent, persistent home for the data which might’ve been called secondary storage at that time. That storage would’ve ranged from the earliest punch card technologies to tape to hard-disk drives.