# Glossary

Availability Zone

  Availability zones are a logical subdivision of cloud block storage, compute and network services. They provide a way for cloud operators to logically segment their compute based on arbitrary factors like location (country, datacenter, rack), network layout and/or power source.

  For more information, refer to Host aggregates.

Boot From Volume

  A server that is created with a Block Device Mapping with `boot_index=0` and `destination_type=volume`. The root volume can already exist when the server is created or be created by the compute service as part of the server creation. Note that a server can have volumes attached and not be boot-from-volume. A boot from volume server has an empty (“”) `image` parameter in `GET /servers/{server_id}` responses.

Cell

  A cell is a shard or horizontal partition in a nova deployment. A cell mostly consists of a database, queue, and set of compute nodes. All deployments willl have at least one cell (and one “fake” cell). Larger deployments can have many.

  For more information, refer to Cells (v2).

Cross-Cell Resize

  A resize (or cold migrate) operation where the source and destination compute hosts are mapped to different cells. By default, resize and cold migrate operations occur within the same cell.

  For more information, refer to Cross-cell resize.

Host Aggregate

  Host aggregates can be regarded as a mechanism to further partition an Availability Zone; while availability zones are visible to users, host aggregates are only visible to administrators. Host aggregates provide a mechanism to allow administrators to assign key-value pairs to groups of machines. Each node can have multiple aggregates, each aggregate can have multiple key-value pairs, and the same key-value pair can be assigned to multiple aggregates.

  For more information, refer to Host aggregates.

Same-Cell Resize

  A resize (or cold migrate) operation where the source and destination compute hosts are mapped to the same cell. Also commonly referred to as “standard resize” or simply “resize”. By default, resize and cold migrate operations occur within the same cell.

  For more information, refer to Resize and cold migrate.

this page last updated: 2026-06-10 20:07:09

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