Provisioning in Trident has two primary phases. The first of these associates a storage class with the set of suitable backend storage pools and occurs as a necessary preparation before provisioning. The second encompasses the volume creation itself and requires choosing a storage pool from those associated with the pending volume's storage class. This section explains both of these phases and the considerations involved in them, so that users can better understand how Trident handles their storage.
Associating backend storage pools with a storage class relies on both the storage class's requested attributes and its storagePools, additionalStoragePools, and excludeStoragePools lists. When a user creates a storage class, Trident compares the attributes and pools offered by each of its backends to those requested by the storage class. If a storage pool's attributes and name match all of the requested attributes and pool names, Trident adds that storage pool to the set of suitable storage pools for that storage class. In addition, Trident adds all storage pools listed in the additionalStoragePools list to that set, even if their attributes do not fulfill all or any of the storage class's requested attributes. Use the excludeStoragePools list to override and remove storage pools from use for a storage class. Trident performs a similar process every time a user adds a new backend, checking whether its storage pools satisfy those of the existing storage classes and removing any that have been marked as excluded.
Trident then uses the associations between storage classes and storage pools to determine where to provision volumes. When a user creates a volume, Trident first gets the set of storage pools for that volume's storage class, and, if the user specifies a protocol for the volume, it removes those storage pools that cannot provide the requested protocol (a HCI/SolidFire backend cannot provide a file-based volume while an ONTAP NAS backend cannot provide a block-based volume, for instance). Trident randomizes the order of this resulting set, to facilitate an even distribution of volumes, and then iterates through it, attempting to provision the volume on each storage pool in turn. If it succeeds on one, it returns successfully, logging any failures encountered in the process. Trident returns a failure if and only if it fails to provision on all the storage pools available for the requested storage class and protocol.