OpenStack database nova nova_cell0 difference
In OpenStack, nova and nova_cell0 are integral parts of the Nova service architecture, particularly in a deployment that uses cells. Here's a breakdown of their differences and purposes:
Nova Database
- The Nova database is the primary database for the Nova service.
- It stores global information related to instances, flavors, host aggregates, availability zones, and more.
- Data in the Nova database is structured to manage and coordinate resources across all cells in a multi-cell setup.
- It is essential for the overall orchestration and high-level state management of instances across the OpenStack cloud.
Nova Cell0 Database
- The
nova_cell0 database is specific to the cell0 construct introduced in Nova with the Cells v2 architecture. - It acts as a placeholder database for instances that fail scheduling (i.e., instances that cannot be mapped to any host or cell).
- When an instance cannot be scheduled (for example, due to lack of resources), its metadata is recorded in
nova_cell0. This helps to ensure consistent tracking of such instances, even if they are not successfully deployed.
Key Differences
| Feature | Nova Database | Nova Cell0 Database |
|---|
| Purpose | Central database for global Nova service information. | Placeholder for failed-to-schedule instances. |
| Scope | Manages all cells and global state across OpenStack. | Specific to tracking unscheduled instances. |
| Usage | Active management of instances, resources, and services. | Records metadata for failed instances. |
| Connection to Cells | Interacts with all cells in a multi-cell setup. | Does not interact with other cells. |
Why is Nova Cell0 Important?
Before the introduction of nova_cell0, unscheduled instances could lead to inconsistencies or orphaned records in OpenStack environments. nova_cell0 ensures:
- Consistency in the database structure.
- A clear audit trail for instances that failed scheduling.
- Improved debugging and troubleshooting for deployment issues.
This distinction is particularly important in large-scale OpenStack deployments that use Cells v2, where each cell typically has its own database and message queue, but nova_cell0 remains a global construct.
