A swing shelf (temporary external capacity) is required during a SAS-to-NVMe (Evergreen) upgrade specifically when there is insufficient free space elsewhere in the array to evacuate the data residing in the legacy chassis.
The Evergreen "Stateless" or XFORM upgrade involves replacing the legacy SAS-based controller chassis (which contains SAS SSDs) with a new NVMe-based FlashArray//X chassis (which uses DirectFlash Modules).
* Evacuation Requirement: Before the old chassis can be removed, the data on its internal drives must be moved to a safe location. If the customer already has external SAS expansion shelves with enough free space to hold this data, Purity will transparently migrate the data there, and no swing shelf is needed.
* Swing Shelf Scenario: If the array is highly utilized (e.g., the chassis is full and external shelves are full or non-existent), the Implementation Engineer must attach a temporary "swing shelf" provided by Pure Storage. This shelf acts as the evacuation target. Once the data is moved to the swing shelf, the old chassis is replaced. The data is then migrated back from the swing shelf to the new NVMe media in the new chassis, and the swing shelf is returned.