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Storage architecture
OpenStack has multiple storage realms to consider:
- Block Storage (cinder)
- Object Storage (swift)
- Image storage (glance)
- Ephemeral storage (nova)
- Filesystem storage (manila)
Block Storage (cinder)
The Block Storage (cinder) service manages volumes on storage devices
in an environment. In a production environment, the device presents
storage via a storage protocol (for example, NFS, iSCSI, or Ceph RBD) to
a storage network (br-storage) and a storage management API
to the management network (br-mgmt). Instances are
connected to the volumes via the storage network by the hypervisor on
the Compute host.
The following diagram illustrates how Block Storage is connected to instances.
| 1. | A volume is created by the assigned cinder-volume
service using the appropriate cinder driver.
The volume is created by using an API that is presented to the
management network. |
| 2. | After the volume is created, the nova-compute service
connects the Compute host hypervisor to the volume via the storage
network. |
| 3. | After the hypervisor is connected to the volume, it presents the volume as a local hardware device to the instance. |
Important
The LVMVolumeDriver
is designed as a reference driver implementation, which we do not
recommend for production usage. The LVM storage back-end is a
single-server solution that provides no high-availability options. If
the server becomes unavailable, then all volumes managed by the
cinder-volume service running on that server become
unavailable. Upgrading the operating system packages (for example,
kernel or iSCSI) on the server causes storage connectivity outages
because the iSCSI service (or the host) restarts.
Because of a limitation
with container iSCSI connectivity, you must deploy the
cinder-volume service directly on a physical host (not into
a container) when using storage back ends that connect via iSCSI. This
includes the LVMVolumeDriver
and many of the drivers for commercial storage devices.
Note
The cinder-volume service does not run in a highly
available configuration. When the cinder-volume service is
configured to manage volumes on the same back end from multiple hosts or
containers, one service is scheduled to manage the life cycle of the
volume until an alternative service is assigned to do so. This
assignment can be made through the cinder-manage
CLI tool. This configuration might change if cinder
volume active-active support spec is implemented.
Object Storage (swift)
The Object Storage (swift) service implements a highly available, distributed, eventually consistent object/blob store that is accessible via HTTP/HTTPS.
The following diagram illustrates how data is accessed and replicated.
Image storage (glance)
The Image service (glance) can be configured to store images on a variety of storage back ends supported by the glance_store drivers.
Important
When the File System store is used, the Image service has no
mechanism of its own to replicate the image between Image service hosts.
We recommend using a shared storage back end (via a file system mount)
to ensure that all glance-api services have access to all
images. Doing so prevents losing access to images when an infrastructure
(control plane) host is lost.
The following diagram illustrates the interactions between the Image
service, the storage device, and the nova-compute service
when an instance is created.
| 1 | When a client requests an image, the glance-api service
accesses the appropriate store on the storage device over the storage
network (br-storage) and pulls it into its cache. When the
same image is requested again, it is given to the client directly from
the cache. |
| 2 | When an instance is scheduled for creation on a Compute host, the
nova-compute service requests the image from the
glance-api service over the management network
(br-mgmt). |
| 3 | After the image is retrieved, the nova-compute service
stores the image in its own image cache. When another instance is
created with the same image, the image is retrieved from the local base
image cache. |
Ephemeral storage (nova)
When the flavors in the Compute service are configured to provide
instances with root or ephemeral disks, the nova-compute
service manages these allocations using its ephemeral disk storage
location.
In many environments, the ephemeral disks are stored on the Compute host's local disks, but for production environments we recommend that the Compute hosts be configured to use a shared storage subsystem instead. A shared storage subsystem allows quick, live instance migration between Compute hosts, which is useful when the administrator needs to perform maintenance on the Compute host and wants to evacuate it. Using a shared storage subsystem also allows the recovery of instances when a Compute host goes offline. The administrator is able to evacuate the instance to another Compute host and boot it up again. The following diagram illustrates the interactions between the storage device, the Compute host, the hypervisor, and the instance.
| 1 | The Compute host is configured with access to the storage device.
The Compute host accesses the storage space via the storage network
(br-storage) by using a storage protocol (for example, NFS,
iSCSI, or Ceph RBD). |
| 2 | The nova-compute service configures the hypervisor to
present the allocated instance disk as a device to the instance. |
| 3 | The hypervisor presents the disk as a device to the instance. |
Filesystem storage (manila)
The shared filesystem service (manila) can be configured to provide file systems on a variety of storage back ends as supported by the manila_store drivers.
