Secondary Storage: Definition, Devices and How It Can Support Your Backup Strategy

In this blog, we delve deeper into the basics of primary and secondary storage, how they differ from one another and how secondary storage specifically is a significant factor in organizations’ overall data backup and disaster recovery strategy.

What is secondary storage?

Secondary storage, sometimes termed auxiliary storage, refers to the storage of data that is not accessed frequently as the data in primary storage. It is a non-volatile memory medium that preserves data until and unless it has been deleted or overwritten. Secondary storage can be hosted on-premises, on an external device or in the cloud. With a variety of media types available, secondary storage enables organizations to store data ranging from a few megabytes to petabytes.

Why is secondary storage used?

The main function of secondary storage is to complement primary storage. Data that doesn’t need primary storage gets migrated to the secondary storage devices, in turn freeing up space and improving the performance of the primary storage devices.

In particular, organizations use secondary storage for backup and disaster recovery data and archival data. As far as backup is involved, most organizations focus on backing up the critical workloads — data that is accessed regularly or most frequently and is prioritized as part of a disaster recovery plan. However, not all data gets accessed or used on a regular basis. For those, a secondary storage device is ideal since it provides data protection and archiving features at lower costs.

Generally, secondary storage technology is much cheaper than primary storage. This type of storage can work perfectly on economical devices more suitable for long-term storage.

Why is secondary storage important?

Secondary storage is associated with external storage devices that are not directly connected to production servers. Data, by default, get saved to production storage tied to an active workload or application. Keeping all the data in one place can be risky. There always is a possibility of hardware and software getting affected by errors, misconfigurations, malware or other threats. Such events are detrimental to production data and adversely impact an organization. Without a proper backup and disaster recovery strategy, such disasters can be a tremendous blow to the continuity of an organization’s operations.

Storing copies of data on a secondary storage platform that is isolated from the production environment and network is instrumental in preventing data loss and ensuring recovery.

Another significant benefit of opting for secondary storage devices is cost-cutting. It provides a low-cost storage tier, although the stored data might not be immediately accessible.

Many organizations use secondary storage as a part of their backup strategy to ensure one copy of the valuable business data is always inaccessible via the internet or internal network.

How can Unitrends help you with secondary storage, backup and recovery?

Unitrends follows a 4-2-2 strategy (4 copies, 2 formats, 2 stored off-site) where the second copy gets stored in a hot state and is immediately available for recovery. Another copy gets stored offline as a “cold copy.” This enables you to carry out disaster recovery in different ways — either by leveraging the cloud or by physically moving the data off-site, or a combination of both.

Unitrends appliances possess replication capabilities that allow them to perform WAN-accelerated replication to move backup copies to an alternate target. Hence, you can transfer data efficiently without using add-ons or proxy servers and gateways.

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What are the three main types of secondary storage?

Secondary storage devices encourage long-term archiving and storing of rarely accessed data in less expensive drives. Devices like compact disks (CDs) and flash drives, which are deemed secondary storage devices, can be used to transfer data from one device to another.

Secondary storage devices can be classified into three broad categories.

Magnetic storage devices

Magnetic storage devices use magnetization process to write, rewrite and access data. Data gets saved here in binary form. These devices store data in the form of tiny, magnetized dots. The dots are created, read and erased using magnetic fields created by tiny electromagnets.

Hard disk drives (HDDs)

They consist of a series of circular disks called platters, arranged one over the other around a spindle. The disks are made of non-magnetic materials and coated with magnetic material. A typical modern HDD has a storage capacity measured in terabytes (TB).

Tape drives

Devices that store data on magnetic tape are known as tape drives. These devices allow large volumes of data to be mechanically created, stored for a long time and easily accessed.

Floppy disk drives

This flexible disk has a magnetic coating on it and is packed inside a protective plastic envelope. One of the oldest portable storage devices, floppy disks are not used that much now due to their limited storage capacity.

Optical storage devices

These devices use optical storage technology to read and write data. Data gets stored digitally, and lasers are used in these devices to read and write data.

CD drives

CD drives generally use laser rays to read and write data. With 700 MB of storage space, these devices are cheap, portable and one of the most popular storage devices used.

DVD drives

They can store 15 times the data CDs hold and rich multimedia files that require high storage capacity.

Blue-ray drives

A blue-ray disk can store high-definition (HD) video and other multimedia files. It uses a shorter wavelength laser than CD/DVD and therefore can store more data.

Solid-state storage devices

Solid-state storage devices do not involve magnetic disks or any moving parts. They store data on non-moving components.

Solid-state drives (SSDs)

These devices store data using flash-based memory much faster than traditional hard disks. SSDs have no moving parts and upgrading to one is a great way to speed up your computer and make it more resilient.

USB drives

USB flash drives are essential for quickly moving files from one system to another. They can now hold up to 2TB of storage and are a convenient medium to store and transfer smaller files.

What is the difference between primary and secondary storage?

Both primary and secondary storage is integral to a comprehensive backup strategy. The former provides rapid and efficient access to data, while the latter takes care of archiving massive volumes of data for a longer period. Though they work together to create an ideal backup storage strategy, their purposes differ (see Table 1).

Primary storage

Primary storage refers to the methods and technologies used in storing data that is actively in use and crucial for an organization’s operations. Based on the computing environment, primary storage might consist of hard disks or flash-based SSDs installed locally on an application server or file server. Hard disk media is the most prevalent form of primary storage, but SSDs are gradually replacing them because of their high-performance capabilities. In some cases, both types of storage media (HDDs and SSDs) are employed by primary storage systems.

Secondary storage

This is a permanent, cheaper, larger and slower backup process typically used for long-term storage of cold data. Secondary storage readily supports lengthy retention requirements and other data that may need to be recovered to make the environment whole and running after a major outage. Often considered a lower tier than the primary storage tier, secondary storage is leveraged as a backup storage medium, just like cloud storage, to protect from data loss.

Table 1. Primary vs. secondary storage


Primary Storage

Secondary Storage




Data availability



Data durability


Not necessary

First byte retrieval



Likely storage type


HDD/spinning disk, tape


More expensive

Less expensive

You may leverage a combination of primary and secondary storage to balance recovery needs, retention requirements and costs. This can ultimately improve backup and recovery performance in key areas while reducing the overall cost of storing all your organization’s data.

Major storage vendors usually designate various kinds of storage based on tiered storage plans. In such a structure, primary storage is considered hot storage, where your business-critical data gets stored and is made immediately available for recovery. Warm storage is for data that is required less frequently but needs to be kept close enough for easy access when needed. Warm storage is considered secondary storage.

Although secondary storage is just about any storage not considered primary storage, some organizations even separate the archival data from the secondary storage and store them in a third tier. This is called cold storage or sometimes tertiary or auxiliary storage, often used for non-critical data stored on low-cost media with longer recovery time objectives (RTOs).

The hotter the storage, the quicker you can access your files. However, it comes at a higher cost — either one-off or long-term. Cold storage, often called “cheap and deep” storage, is ideal for situations where data is archived, and you don’t need to access it frequently.

What should you consider when buying secondary storage?

Due to the sheer volume of data generated daily, it is ideal to have a highly resilient and flexible storage option.

When considering storage options, the recovery speed is a critical factor to consider. Depending on the medium, secondary storage may take a much longer time for data retrieval than hot storage.

Cost is another important factor to consider when choosing the storage type. In many cases, secondary storage is about cost savings. There’s no point in paying premium prices for data that doesn’t require instant access.

Slower hard drives and tapes are the ideal mediums for secondary storage for data that requires long retention times and infrequent access. However, while opting for secondary storage, all the disks and tapes must be tested regularly to ensure they function properly. Companies need to monitor device lifespan to retire the aging devices before they fail.

Data should not be kept in secondary storage devices indefinitely. They must be evaluated regularly, preferably yearly, to see if the resources are effectively used.

How does secondary storage support backup and recovery?

Recovery point objective (RPO) and recovery time objective (RTO) are crucial metrics for developing a good data backup and recovery strategy. Many organizations have multiple RPOs and RTOs that reflect each workload’s significance to their business. This helps to classify the workloads into a tiered storage structure, thus determining the level of data protection.

In situations where data is sensitive and mission-critical, the data needs to be backed up to redundant arrays to prevent data loss. Organizations must ensure they can replace the information and applications required to continue business operations seamlessly if they run into issues with their primary storage. That’s why secondary storage is so crucial.

Sectors like education and government, which deal with high volumes of data regularly, depend widely on secondary storage to back up and retain their data long-term since it frees up space on the primary storage and reduces overall storage costs.

For backing up and archiving data, organizations are nowadays also relying on cloud storage. Compared to primary storage, cloud storage can be a more cost-effective tool. Cloud backups are easy to access when connected to the internet.

With cloud backup, organizations access their data stored on physical servers outside their own data centers. The degree of effectiveness of organizations’ cloud-based architecture has a direct influence over how a recovery plan performs. To measure and optimize the performance of recovery plans, RPO and RTO requirements of organizations are essential. Unitrends Disaster Recovery-as-a-Service (DRaaS) ensures automatic testing of recoveries weekly or monthly at the application level, along with proper documentation of the RTOs and RPOs. Hence, organizations can meet their RTOs successfully.

For organizations, regular storage of data is a serious concern. The more the data increases, the more will be the migration of data from primary to secondary storage devices, which will eventually reach a saturation point.


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