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2.6. About Secondary Storage

Secondary storage is associated with a zone, and it stores the following:
The items in zone-based NFS secondary storage are available to all hosts in the zone. CloudStack manages the allocation of guest virtual disks to particular primary storage devices.
To make items in secondary storage available to all hosts throughout the cloud, you can add OpenStack Object Storage (Swift, swift.openstack.org) in addition to the zone-based NFS secondary storage. When using Swift, you configure Swift storage for the entire CloudStack, then set up NFS secondary storage for each zone as usual. The NFS storage in each zone acts as a staging area through which all templates and other secondary storage data pass before being forwarded to Swift. The Swift storage acts as a cloud-wide resource, making templates and other data available to any zone in the cloud. There is no hierarchy in the Swift storage, just one Swift container per storage object. Any secondary storage in the whole cloud can pull a container from Swift at need. It is not necessary to copy templates and snapshots from one zone to another, as would be required when using zone NFS alone. Everything is available everywhere.