Chapter 1. Introduction to Persistent Storage in OpenStack

In general, a clustered storage solution provides greater back end scalability. For example, when using Red Hat Ceph as a Block Storage back end, you can scale storage capacity and redundancy by adding more Ceph OSD (Object Storage Daemon) nodes. Both Block Storage and Object Storage services support Red Hat Ceph as a back end.

The Block Storage service can use multiple storage solutions as discrete back ends. At the back end level, you can scale capacity by adding more back ends and restarting the service. The Block Storage service also features a large list of supported back end solutions, some of which feature additional scalability features.

By default, the Object Storage service uses the file system on configured storage nodes, and can use as much space as is available. The Object Storage service supports the XFS and ext4 file systems, and both can be scaled up to consume as much available underlying block storage. You can also scale capacity by adding more storage devices to the storage node.

The Shared File System Service provisions shares backed by storage from a separate storage pool. This pool (which is typically managed by a third-party back end service) provides the share with storage at the file system level. The Shared File System Service can use both NetApp and CephFS, which can be configured to use a storage pool of pre-created volumes which provisioned shares can use for storage. In either deployment, scaling involves adding more volumes to the pool.

Volumes are consumed only through instances, and can only be attached to and mounted within one instance at a time. Users can create snapshots of volumes, which can be used for cloning or restoring a volume to a previous state (see Section 1.4, “Redundancy and Disaster Recovery”). The Block Storage service also allows you to create volume types, which aggregate volume settings (for example, size and back end) that can be easily invoked by users when creating new volumes. These types can be further associated with Quality-of-Service specifications, which allow you to create different storage tiers for users.

Like volumes, shares are consumed through instances. However, shares can be directly mounted within an instance, and do not need to be attached through the dashboard or CLI. Shares can also be mounted by multiple instances simultaneously. The Shared File System service also supports share snapshots and cloning; you can also create share types to aggregate settings (similar to volume types).

Objects in a container are accessible via API, and can be made accessible to instances and services within the cloud. This makes them ideal as object repositories for services; for example, the Image service (openstack-glance) can store its images in containers managed by the Object Storage service.

The Block Storage service provides basic data security through volume encryption. With this, you can configure a volume type to be encrypted through a static key; the key will then be used for encrypting all volumes created from the configured volume type. See Section 2.2.5, “Configure Volume Encryption” for more details.

Object and container security, on the other hand, is configured at the service and node level. The Object Storage service provides no native encryption for containers and objects. Rather, the Object Storage service prioritizes accessibility within the cloud, and as such relies solely on the cloud’s network security in order to protect object data.

The Shared File System service can secure shares through access restriction, whether by instance IP, user/group, or TLS certificate. In addition, some Shared File System service deployments can feature a separate share servers to manage the relationship between share networks and shares; some share servers support (or even require) additional network security. For example, a CIFS share server requires the deployment of an LDAP, Active Directory, or Kerberos authentication service.

The Block Storage service features volume backup and restoration, providing basic disaster recovery for user storage. Backups allow you to protect volume contents. On top of this, the service also supports snapshots; aside from cloning, snapshots are also useful in restoring a volume to a previous state.

In a multi-backend environment, you can also migrate volumes between back ends. This is useful if you need to take a back end offline for maintenance. Backups are typically stored in a storage back end separate from their source volumes to help protect the data. This is not possible, however, with snapshots, as snapshots are dependent on their source volumes.

The Block Storage service also supports the creation of consistency groups, which allow you to group volumes together for simultaneous snapshot creation. This, in turn, allows for a greater level of data consistency across multiple volumes. See Section 2.2.7, “Configure and Use Consistency Groups” for more details.

Finally, the Block Storage service also features volume replication. This allows you to configure volumes to replicate content between each other, thereby providing basic redundancy.

The Object Storage service provides no built-in backup features. As such, all backups must be performed at the file system or node level. The service, however, features more robust redundancy and fault tolerance; even the most basic deployment of the Object Storage service replicates objects multiple times. You can use failover features like dm-multipath to enhance redundancy.

The Shared File System service provides no built-in backup features for shares, but it does allow you to create snapshots for cloning and restoration.