Cloud storage is a model of networked enterprise storage where data is stored in virtualized pools of storage which are generally hosted by third parties. Hosting companies operate large data centers, and people who require their data to be hosted buy or lease storage capacity from them. The data center operators, in the background, virtualize the resources according to the requirements of the customer and expose them as storage pools, which the customers can themselves use to store files or data objects. Physically, the resource may span across multiple servers and multiple locations. The safety of the files depends upon the hosting companies, and on the applications that leverage the cloud storage.
Cloud storage services may be accessed through a web service application programming interface (API) or by applications that utilize the API, such as cloud desktop storage, a cloud storage gateway or Web-based content management systems.
Cloud computing is believed to have been invented by Joseph Carl Robnett Licklider in the 1960s. However, Kurt Vonnegut refers to a cloud "that does all the heavy thinking for everybody" in his book Sirens of Titan (1959). Since the Sixties, cloud computing has developed along a number of lines; Web 2.0 is the most recent evolution. However, since the internet only started to offer significant bandwidth in the Nineties, cloud computing for the masses has been something of a late developer.
Cloud storage is based on highly virtualized infrastructure and has the same characteristics as cloud computing in terms of agility, scalability, elasticity and multi-tenancy, and is available both off-premises (Amazon S3) and on-premises (ViON Capacity Services)
It is difficult to pin down a canonical definition of cloud storage architecture, but object storage is reasonably analogous. Cloud storage software like Openstack Swift, cloud storage products like EMC Atmos and Hitachi Content Platform, and distributed storage research projects like OceanStore or VISION Cloud  are all examples of object storage and infer the following guidelines.
Cloud storage is:
- Made up of many distributed resources, but still acts as one - often referred to as federated storage clouds 
- Highly fault tolerant through redundancy and distribution of data
- Highly durable through the creation of versioned copies
- Typically eventually consistent with regard to data replicas 
- Companies need only pay for the storage they actually use, typically an average of consumption during a month. This does not mean that cloud storage is less expensive, only that it incurs operating expenses rather than capital expenses.
- Organizations can choose between off-premise and on-premise cloud storage options, or a mixture of the two options, depending on relevant decision criteria that is complementary to initial direct cost savings potential; for instance, continuity of operations (COOP), disaster recovery (DR), security (PII, HIPAA, SARBOX, IA/CND), and records retention laws, regulations, and policies.
- Storage availability and data protection is intrinsic to object storage architecture, so depending on the application, the additional technology, effort and cost to add availability and protection can be eliminated.
- Storage maintenance tasks, such as purchasing additional storage capacity, are offloaded to the responsibility of a service provider.
- Cloud storage provides users with immediate access to a broad range of resources and applications hosted in the infrastructure of another organization via a web service interface.
- Cloud storage can be used for copying virtual machine images from the cloud to on-premise locations or to import a virtual machine image from an on-premise location to the cloud image library. In addition, cloud storage can be used to move virtual machine images between user accounts or between data centers.
Attack surface area
- When data is distributed it is stored at more locations increasing the risk of unauthorised physical access to the data. For example, in cloud based architecture, data is replicated and moved frequently so the risk of unauthorised data recovery increases dramatically. (e.g. disposal of old equipment, reuse of drives, reallocation of storage space) The manner that data is replicated depends on the service level a customer chooses and on the service provided. Different cloud vendors offer different service levels. Risk of unauthorized access to data can be mitigated through the use of encryption, which can be applied to data as part of the storage service or by on-premises equipment that encrypts data prior to uploading it to the cloud.
- The number of people with access to the data who could be compromised (i.e. bribed, or coerced) increases dramatically. A single company might have a small team of administrators, network engineers and technicians, but a cloud storage company will have many customers and thousands of servers and therefore a much larger team of technical staff with physical and electronic access to almost all of the data at the entire facility or perhaps the entire company. Encryption keys that are kept by the service user, as opposed to the service provider limit the access to data by service provider employees.
- It increases the number of networks over which the data travels. Instead of just a local area network (LAN) or storage area network (SAN), data stored on a cloud requires a WAN (wide area network) to connect them both.
- By sharing storage and networks with many other users/customers it is possible for other customers to access your data. Sometimes because of erroneous actions, faulty equipment, a bug and sometimes because of criminal intent. This risk applies to all types of storage and not only cloud storage. The risk of having data read during transmission can be mitigated through encryption technology. Encryption in transit protects data as it is being transmitted to and from the cloud service. Encryption at rest protects data that is stored at the service provider. Encrypting data in an on-premises cloud service on-ramp system can provide both kinds of encryption protection.
Companies are not permanent and the services and products they provide can change. Outsourcing data storage to another company needs careful investigation and nothing is ever certain. Contracts set in stone can be worthless when a company ceases to exist or its circumstances change. Companies can:
- Go bankrupt.
- Expand and change their focus.
- Be purchased by other larger companies.
- Be purchased by a company headquartered in or move to a country that negates compliance with export restrictions and thus necessitates a move.
- Suffer an irrecoverable disaster.
- Performance for outsourced storage is likely to be lower than local storage, depending on how much a customer is willing to spend for WAN bandwidth
- Reliability and availability depends on wide area network availability and on the level of precautions taken by the service provider.. Reliability should be based on hardware as well as various algorithms used.
- Security of stored data and data in transit may be a concern when storing sensitive data at a cloud storage provider
- Users with specific records-keeping requirements, such as public agencies that must retain electronic records according to statute, may encounter complications with using cloud computing and storage. For instance, the U.S. Department of Defense designated the Defense Information Systems Agency (DISA) to maintain a list of records management products that meet all of the records retention, personally identifiable information (PII), and security (Information Assurance; IA) requirements
- Cloud storage is a rich resource for both hackers and national security agencies.
- Piracy and copyright infringement may be enabled by sites that permit filesharing. For example, the CodexCloud ebook storage site has faced litigation from the owners of the intellectual property uploaded and shared there, as have the GrooveShark and YouTube sites it has been compared to.
- The legal aspect, from a regulatory compliance standpoint, is of concern when storing files domestically and especially internationally.
- Cloud collaboration
- Cloud Data Management Interface, better known as CDMI*Cloud collaboration
- Cloud database
- Comparison of online backup services
- File hosting service
- Object storage
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