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 Storage concepts

The OpenStack stack uses the following storage types:

Table 1.2. Storage types
On-instance / ephemeral Block storage (cinder) Object Storage (swift)
Runs operating systems and provides scratch space Used for adding additional persistent storage to a virtual machine (VM) Used for storing virtual machine images and data
Persists until VM is terminated Persists until deleted Persists until deleted
Access associated with a VM Access associated with a VM Available from anywhere
Implemented as a filesystem underlying OpenStack Compute Mounted via OpenStack Block Storage controlled protocol (for example, iSCSI) REST API
Administrator configures size setting, based on flavors Sizings based on need Easily scalable for future growth
Example: 10 GB first disk, 30 GB/core second disk Example: 1 TB "extra hard drive" Example: 10s of TBs of data set storage

You should note that:

  • You cannot use OpenStack Object Storage like a traditional hard drive. The Object Storage relaxes some of the constraints of a POSIX-style file system to get other gains. You can access the objects through an API which uses HTTP. Subsequently you don't have to provide atomic operations (that is, relying on eventual consistency), you can scale a storage system easily and avoid a central point of failure.

  • The OpenStack Image Service is used to manage the virtual machine images in an OpenStack cluster, not store them. It provides an abstraction to different methods for storage - a bridge to the storage, not the storage itself.

  • The OpenStack Object Storage can function on its own. The Object Storage (swift) product can be used independently of the Compute (nova) product.

Command-line clients and other interfaces

swift client

Enables users to submit commands to the REST API through a command-line client authorized as either a admin user, reseller user, or swift user.


Script that initializes the building of the ring file, takes daemon names as parameter and offers commands. Documented in



Storage ring build and rebalance utility. Documented in

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