SPI is a document file generated by ShadowProtect, an application for automated backup copy creation. Unlike similar file format SPF used by the same application, SPI file has differnet purpose and file size. SPI files are used for storing data that changed since the last backup.

Using SPI algorithm is faster and more efficent as only modifed data is indexed the backup. ShadowProtect is a paid software for semiprofessional users.