secondary storage

Secondary storage is storage for noncritical data that will not be frequently accessed. While primary storage typically requires costly, high-performance storage systems, secondary storage systems can function effectively on economical, lower-performing devices that are more appropriate long-term storage. Data that is accessed less frequently can be migrated to secondary storage devices to free up space, improve performance on primary storage devices and lower overall storage costs. Types of secondary storage include devices that can accommodate noncritical data, backup and disaster recovery (DR) data sets, and archival data. Secondary storage may also be referred to as auxiliary storage.

Three types of secondary storage

Secondary storage initially referred to the class of media used to store the data. Today, the term is also about the management of noncritical data rather than the hardware on which that data resides. While secondary storage does not need to be accessed as frequently as primary data, recovery of the data can prove crucial to replacing the information and applications that a business needs.

Generally, there are three types of secondary storage. Each has its own usage characteristics that may determine the type of storage media and storage system that is best suited to the particular task.

  1. Backup and DR. Backup and DR data may reside on a variety of media and systems, usually determined by its volume and how easily and quickly it can be restored. Both processes rely on restoring secondary data to recreate files and applications lost because of user error, malicious attacks or natural events, such as hurricanes, earthquakes and fires. In situations where data is highly sensitive or mission-critical, it may be backed up to redundant arrays to ensure against data loss.
  2. Archival. Archival data is information that is no longer accessed with any regularity but must be maintained and be accessible if needed, such as data related to internal governance or legal compliance regulations. Because access to archival data is infrequent and doesn't require immediate turnaround, archival storage systems -- e.g., optical storage or removable magnetic media, such as tape -- may not be online at all times.
  3. Noncritical active data. Many companies have stored data that may not be accessed very frequently but still needs to be close at hand when needed. Some examples of noncritical active data may include recent documents, database records and emails. This data may be maintained on less expensive, lower-performing storage, but it must be online and readily available. So, tape or optical wouldn't be appropriate for this use case.

Benefits of secondary storage

There are two main benefits to moving infrequently used or backup and DR data from primary storage to secondary storage: to free capacity on primary storage and to lower overall storage costs. An additional benefit is that a secondary storage system can be isolated from the main computing network or situated at a remote site to provide an additional layer of security.

Secondary storage provides a lower-cost, higher-capacity storage tier than primary storage, although the data stored may not be as immediately accessible. This tradeoff is worthwhile in some cases, such as for a backup disk appliance or cloud-based backup. Backup appliances and cloud backup services can store vast amounts of data, but getting access to it requires dedicated backup software. Similarly, optical discs and backup tapes must first be mounted onto their respective libraries before they can be read.

Data sets stored on secondary storage include backup data, test/development data, reference data, archived data and other older operational data that no longer requires daily access. Organizations may still gain value from this data by running analytics against it, or it may need to be stored only to meet regulatory requirements.

Data is often archived for long-term preservation. Whether to meet regulatory compliance or maintain business transaction records, some businesses need to store data for years or indefinitely. Because this data isn't frequently accessed or changed, it is more cost-efficient to store it on high-capacity secondary storage.

Some businesses store archival data in a third tier, separate from the storage tier used for backup and accessed even less frequently. This is called cold storage -- or, sometimes, tertiary storage -- but it is important to note that secondary storage is a blanket term for all nonprimary storage.

Secondary storage vs. primary storage

Secondary storage data usually resides on devices with non-volatile memory (NVM), such as hard disk drives (HDDs), solid-state drives (SSDs), tape drives, Optical media and cloud storage. The devices are typically used to protect data for DR or for long-term retention.

Secondary storage is considered a lower tier than the primary storage tier. For backup and archival use, secondary storage may not be under the direct control of a server's operating system (OS). Also, in some cases, secondary storage devices may not interact directly with an application.

Primary vs. secondary storage
Comparison table of primary and secondary storage devices

Primary storage, also called active storage, refers to a storage tier containing frequently accessed, mission-critical applications and their data. This tier of storage could take the form of local disks or SSDs installed inside a server's chassis or as disks or SSDs in an external storage array.

Secondary storage is often referred to as Tier 2 storage, with primary storage serving as Tier 1 and, sometimes, Tier 0 storage, particularly when referring to storage systems that use SSDs or if computer memory is being employed as a storage layer.

The term primary storage was also once used to refer to volatile memory, such as random access memory (RAM) or cache memory, which is cleared when a device is powered off. NVM acts like magnetic media, maintaining the data even when powered off. It is sometimes referred to as flash memory and is typically packaged as an SSD, as a Universal Serial Bus (USB) flash drive or in newer form factors for memory cards, such as M.2.

Examples of secondary storage devices

Secondary storage backs up primary storage by copying data through replication or other data protection and recovery methods using backup software or storage system snapshots.

External HDDs are portable devices that serve as secondary computer storage or as a network drive. An external drive attaches to a computer via a standard USB port.

External hard drive
External hard drive

Enterprises seldom deploy consumer-oriented portable devices as secondary storage due to concerns about data security and capacity. They use portable storage devices that integrate enterprise-class data encryption at the device or cartridge level, which prevents unauthorized users from gaining access to the data.

Quantum Scalar i3
Quantum Scalar i3 is a tape storage device. Tape is an older medium for secondary storage that is still widely used today because of its high capacities and portability.

Other media used for enterprise secondary storage include disk-based systems or magnetic tape libraries. When performance of a secondary storage system is important, flash media, such as SSDs, can be paired with HDDs in a hybrid flash environment, such as hyper-converged storage for secondary copy data.

Some all-flash arrays (AFAs) support replication to third-party disk systems for converged data protection in a tiered storage environment.

Due to its comparably higher cost and lower write endurance, all-flash storage is rarely used exclusively for secondary data.

Pure Storage FlashBlade enterprise storage array
A Pure Storage FlashBlade enterprise storage array

In a business environment, an older network-attached storage (NAS) box, storage area network (SAN) or tape library can potentially serve as secondary storage. More recently, object storage devices have been used in secondary storage to lessen the demands on primary storage arrays.

Asigra TrueNAS M-Series backup appliance
The Asigra TrueNAS M-Series backup appliance, like all backup appliances, is secondary storage.

Cloud as a secondary storage tier

The rise of the software as a service (SaaS) model expands cloud storage use cases to provide a secondary or tertiary tier. This is especially true when cloud storage is used for backup and data archiving.

Cloud-based archiving has emerged as a cost-effective tool to store older data that rarely changes, in comparison to primary storage in a server. A secondary storage system internally managed in an on-premises data center may function as a private cloud.

By contrast, data packets shipped via broadband internet pipes to a third-party service provider reside in a public cloud, such as Amazon Web Services (AWS) or Microsoft Azure. Companies frequently choose a hybrid cloud model that keeps some data locally and archives less active data sets in a public cloud repository.

Public cloud storage consumers access data stored on physical servers outside of their own data center, connecting to it via the internet. This enables data to be accessed from any device, although customers may incur charges above the monthly cloud subscription for ingress and egress and for running operations on the data.

For those reasons, plus lingering concerns about data security and availability, many enterprise customers take a cautious approach when selecting the public cloud as a secondary target, although adoption of cloud secondary storage is accelerating. The SaaS model enables a company to scale its cloud-based consumption costs based on varying demand.

This was last updated in January 2020

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