Medium Xtrabackup currently already copies ib_buffer_pool file if present:
180608 15:30:43 [00] Compressing ib_buffer_pool to /var/lib/xtrabackup_backupfiles/ib_buffer_pool.qp
180608 15:30:43 [00] ...done
180608 15:30:43 Backup created in directory
It will be a nice feature if we could instruct the tool to make MySQL update that file with latest pages present on BP. This is done via SET GLOBAL innodb_buffer_pool_dump_now=ON .
This feature is especially useful for PXC when doing SST. Joiner will have it's BP warm-up right after it joins the cluster.