With reduced lock feature, currently the files discovered or available at the copy phase are copied. For large files that under go DDLs like truncate or DROP, xtrabackup copies the IBD file that is deleted by Server. The optimization is to avoid copying the deleted files by Server. after copying every chunk, xtrabackup has information (gathered from redo) that a file is deleted or not. This is a best effort only since redo log processing might be slow. We have mechanisms to handle the deleted files.

Avoid copying the deleted files. This can result in faster and smaller backups.