Technology is disclosed for a data migration process for a storage server that preserves storage efficiency information. The storage server receives an indication of a group of two or more files and selects among the two or more files a reference file and one or more selected files. The storage server initiates a first migration operation to copy or move the reference file from a source storage server to a destination storage server. The storage server initiates one or more additional migration operations to copy or move the selected files from the source storage server to the destination storage server. At least one of the additional migration operations include a step of transmitting to the destination storage server data blocks of the selected files that are not shared between the reference file and the selected files, but avoid transmitting to the destination storage server the blocks shared with the reference file.