The term secondary storage can have a few definitions, depending on its context.
Definition 1: Secondary storage, sometimes called auxiliary storage or external storage, is non-volatile storage that is not under the direct control of a computer's central processing unit (CPU) or does not directly interact with an application.
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Typically, secondary storage is used to back up primary storage through replication or other data protection methods, which involves holding a secondary copy of the data. In a business environment, an older network-attached storage (NAS) box, storage-area network (SAN) or tape library may be used for secondary storage. Object storage devices can also be used to implement secondary storage and lessen the demand on primary storage arrays. The growth of corporate data has prompted many storage managers to move data to secondary storage to ease the strain on primary storage systems to reclaim more expensive storage arrays and maintain older data in an easily accessible form to satisfy business and regulatory compliance requirements.
The data in secondary storage may not be as current as the data in primary storage, especially when backups are not policy-driven or automated. Secondary storage can also refer to data that is less frequently accessed than primary or production data. Because it is accessed less frequently, it can be migrated to secondary storage devices that perform at a lower level than primary storage and are less expensive. For many companies, placing a second class of storage between primary storage and archival storage is the first step toward a tiered storage environment.
Definition 2: Computer storage is made up of primary and secondary storage. Primary storage typically refers to random access memory (RAM) placed near the computer’s CPU to reduce the amount of time it takes to move data between the storage and CPU.
Secondary storage (sometimes referred to as secondary memory) is at the lower level of the storage hierarchy. It commonly refers to hard disk drives (HDDs), solid-state drive (SSD) storage (flash) or other types of storage devices.
Computers use primary and secondary storage for a number of reasons:
- RAM-based storage is volatile. This means RAM contents are not kept when the power is turned off. In contrast, non-volatile storage devices such as HDDs and SSDs retain their contents even without power.
- RAM is far more expensive than non-volatile storage on a cost-per-gigabyte basis. The tradeoff is that primary storage (RAM) is much faster than secondary storage. A combination of primary and secondary data allows the computer to take advantage of high-speed, low-capacity, volatile storage and high-capacity, relatively low-speed, non-volatile storage.
Definition 3: When primary storage refers to local disks installed inside the server’s chassis or to disks in an external storage array, secondary storage may refer to offline or remote storage (such as optical media, a backup tape or a remote archive).
Secondary storage typically provides high capacity, but the data may not be immediately accessible. A backup server, for example, may contain vast quantities of data, but the data set is only accessible through dedicated backup software, not through file-level read operations. Similarly, secondary storage media such as optical disks and backup tapes must be mounted before they can be read.
Definition 4: Secondary storage can also refer to cloud storage. This is especially true when cloud storage is used for data archiving. Many organizations find cloud-based archives to be a more cost-effective way to store aging, relatively static data than locally on a server’s primary storage.