When a share is created, it will automatically begin migrating in the background, in addition to servicing inline requests. This migration is controlled by the shadow migration service. There is a single global tunable, which is the number of threads dedicated to this task. Increasing the number of threads will result in greater parallelism at the expense of additional resources.
The shadow migration service can be disabled, but this should only be used for testing purposes, or when the active of shadow migration is overwhelming the system to the point where it needs to be temporarily stopped. When the shadow migration service is disabled, synchronous requests are still migrated as needed, but no background migration occurs. With the service disabled, no shadow migration will ever complete, even if all the contents of the filesystem are read manually. It is highly recommended to always leave the service enabled.