The space-utilization statistic for the pool (Pct Util) includes the space used by any cached copies of files in the storage pool. The migratable-data statistic (Pct Migr) does not include space occupied by cached copies of files.
The server compares the migratable-data statistic with migration-threshold parameters to determine when migration should begin or end. For more information about storage pool statistics, see Monitoring storage-pool and volume usage.