Every virtual machine (VM) instance stores its metadata on a metadata server. Your VM automatically has access to the metadata server API without any additional authorization. Compute Engine maintains the metadata keys and values for your VMs and projects in directories. Each directory stores metadata entries in the form of key-value pairs. Some directories contain subdirectories.

The metadata server is particularly useful when used in combination with startup and shutdown scripts because you can use the metadata server to programmatically get unique information about a VM, without additional authorization.

Project metadata is defined at project scope and provides information that applies to all VMs in a project. When you set this metadata, the metadata entries propagate to all VMs in that project.

Zonal metadata is defined at a zonal scope within a project and provides information about VMs in that specific zone in that project. When you set zonal metadata, the metadata entries propagate to all VMs in that configured zone in that project. When compared to project metadata, zonal metadata helps you with fault isolation and provides greater reliability.