The shadow migration service allows for automatic migration of data from external or internal sources. This functionality is described in great detail in Chapter 14, Shadow Migration. The service itself only controls automatic background migration. Regardless of whether the service is enabled or not, data will be migrated synchronously for in-band requests.
The service should only be disabled for testing purposes, or if the load on the system due to shadow migration is too great. When disabled, no filesystems will ever finish migrating. The primary purpose of the service is to allow tuning of the number of threads dedicated to background migration.