Cloud storage is a relatively new concept that is becoming a more recognizable term in industry vernacular. Originally delivered as a service, it gained early popularity with Web 2.0 startups looking to outsource storage administration. As the concept spread and offerings expanded, the industry has now embraced two flavors of cloud storage: public and private. This article reviews the choices a user faces when choosing a private versus public cloud storage offering. It describes data types and identifies scenarios where cloud storage is a good solution and where it is a poor fit. This article also covers usage patterns, security, performance and cost implications to educate the reader on differences between public and private cloud storage.

Public Versus Private, What Is the Difference?
The difference between private and public storage clouds is simple. Where is the cloud deployed? A public cloud is offered as a service, usually over an Internet connection. Private clouds are deployed inside the firewall and managed by the user organization. Locality is a simple difference that drives very unique experiences and capabilities to the end user.

Public clouds typically charge a monthly usage fee per GB combined with bandwidth transfer charges. Users can scale the storage on-demand and will never need to purchase storage hardware. Service providers manage the infrastructure and pool resources into capacity that any customer can claim.

Private clouds are built from software running on customer-supplied commodity hardware. The storage is typically not shared outside the enterprise and full control is retained by the organization. Scaling the cloud is as simple as adding another server to the pool and the self-managing architecture expands the cloud by adding performance and capacity.

Cloud Storage Is Not for All Types of Data
Storage clouds are designed to support unstructured or file data types. Yet not all file data is the same and not all files are a fit for cloud storage. (Block-based clouds are starting to emerge but leveraging these clouds requires a unique locality of resources and is beyond the scope of this article.) For example, a highly transactional file like an NFS-mounted database requires a class of storage that is beyond either private or public cloud storage offerings. High-speed access to this file would be limited by a public cloud Internet connection. Private clouds are designed for scale and performance of many files on many nodes and would also struggle to support a database. Instead, users should consider files with the following attributes as candidates for cloud storage:

• Larger files with lots of read access: Digital content, streaming media, video, music, etc.
• Parallel streaming writes: Video surveillance (private clouds)
• Long-term storage files: Backup and archival files (private clouds)
• Geographically shared files: Access from different geographies (public clouds)

These all have several aspects in common, including huge data sets and file systems, parallel file serving requirements, longevity of file access, and the need for low-cost deployments.

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