Secondary storage is used to protect inactive data written from a primary storage array to a nonvolatile tier of disk, flash or tape. Secondary storage is synonymous with the terms secondary memory, auxiliary storage and external storage.
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Secondary storage is a trade-off between high performance and economical long-term archiving. Because it is accessed less frequently, data can be migrated to secondary storage devices with lower performance and costs.
Companies are increasingly placing a second class of storage between primary storage and archival storage as the foundation for a tiered storage environment.
Secondary storage vs. primary storage
Secondary storage commonly refers to nonvolatile storage devices, such as hard disk drives (HDDs) and solid-state drives (SSDs), that protect data for disaster recovery or long-term retention. Optical media, backup tapes and remote archives are common secondary storage technologies.
Secondary storage sits below a company's primary storage tier, and is not under the direct control of a computer's central processing unit (CPU). Secondary storage devices do not interact directly with an application.
The purpose of secondary storage is to provide a high-capacity tier, although the data stored is not immediately accessible. For example, a backup server is capable of storing a vast amount of data, but getting access to it requires dedicated backup software. Similarly, optical disks and backup tapes must first be mounted before they can be read.
A backup storage device is a type of secondary storage. Organizations often install multiple physical backup appliances in at least two locations to ensure data is redundant. The emergence of the public cloud as a storage tier has allowed some companies to reduce, if not eliminate, the need for such backup hardware.
Primary storage refers to local disks installed inside a server's chassis, or to disks in an external storage array. Primary storage typically refers to random access memory (RAM) located near a computer's CPU. This placement reduces the time needed to move data between storage and the CPU.
Because RAM is volatile, it holds active data sets as long as the computer is connected to a power source. Secondary storage, by contrast, uses nonvolatile storage devices, such as HDDs and SSDs, which retain their contents even without power. Nonvolatile storage media is also less expensive than RAM on a cost-per-gigabyte basis.
Devices used in a secondary storage tier
External hard drives are portable devices that serve as either secondary computer storage or a network drive. An external drive attaches to a computer via a standard USB port. Older removable media, such as a floppy disk or USB drive, are most often used by consumers to back up personal computer storage. Newer computer systems do not support floppy disks.
Enterprises seldom deploy consumer-oriented portable devices as secondary storage due to concerns about data security and inventory management. 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.
Other media used for enterprise secondary storage include disk-based systems or magnetic tape libraries. 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 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.
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.
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 aged data that rarely changes, in comparison to primary storage in a server. A secondary storage system internally managed in a data center is known as a private cloud.
By contrast, data packets shipped via broadband internet pipes to a third-party services provider reside in a public cloud, such as Amazon Web Services 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 allows 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 pubic cloud as a secondary target. The SaaS model allows a company to scale its cloud-based consumption costs based on varying demand.
Why secondary storage is gaining prominence
Due to corporate data growth, storage managers are adopting more secondary storage to reclaim capacity on primary storage arrays. The ability to maintain older data copies in an easily accessible form satisfies business and regulatory compliance requirements.
The data in secondary storage is usually older than the data in primary storage, especially if backups lack policy-driven automation. The term secondary storage sometimes refers to data that is accessed less frequently than primary or production data.
Several factors have contributed to the growing importance of secondary storage. Rather than simply parking data for the long term, companies are facing mounting pressure to derive greater value from it.
The emergence of big data analytics has companies storing more (and larger) data sets. Amid stepped-up legal requirements, companies are reluctant to delete older data.
Vendors that specialize in secondary storage
In recent years, storage vendors have shifted their attention to boosting software capabilities to enable customers to build tiered secondary and tertiary storage to a cloud, backup product or even to other vendors' storage.
Startups specializing in secondary storage include Cohesity Inc. and Rubrik Inc. Cohesity sells software on a branded appliance to converge archiving, analytics and data protection on one platform. Rubrik provides a hardware platform that converges backup, data reduction and version management.
Customers should consider several requirements for speed and performance when choosing a secondary storage system, such as data ingestion, restore times, archiving and snapshots. Other key features revolve around metadata search and reporting capabilities.