1. A method for migrating data in a computer system that includes multiple processes running on multiple tightly coupled processors having simultaneous access to a file system comprising multiple physical storage volumes, comprising: identifying migration data to be migrated from a source physical storage location in the file system; identifying a target physical storage location in the file system to receive the migration data; creating a track bitmap dividing the migration data into a plurality of tracks and comprising a mapping of the tracks from the source physical storage location to the target physical storage location; creating a runtime bitmap identifying the migration data as located in the source physical storage location; creating a re-migration bitmap for recording runtime write requests to the migration data while the migration is takes place; migrating the migration data from the source physical storage location to the target physical storage location and supporting runtime input/output requests to the migration data while the migration takes place, the migrating comprising: disallowing new space allocations by the file system to the source physical storage location while the migration takes place; performing a first migration pass where the tracks of the migration data are copied from the source physical storage location to the target physical in accordance with the track bitmap; receiving runtime write requests to the migration data while the first migration pass takes, determining that runtime write requests affect the migration data by reference to the runtime bitmap, servicing the runtime input-output with the migration data stored in the source physical storage location, and recording the migration data affected by the runtime write requests in the re-migration bitmap; and performing a second migration pass where the migration data affected by the runtime write requests identified in the re-migration database are copied from the source physical storage location to the target physical storage location in accordance with the mapping in the track bitmap.