Although xtrabackup does not block your database’s operation, any backup can add
load to the system being backed up. On systems that do not have much spare I/O
capacity, it might be helpful to throttle the rate at which xtrabackup reads and
writes data. You can do this with
xtrabackup --throttle option.
Image below shows how throttling works when
xtrabackup --throttle =1.
By default, there is no throttling, and xtrabackup reads and writes data as quickly as it can. If you set too strict of a limit on the I/O operations, the backup might be so slow that it will never catch up with the transaction logs that InnoDB is writing, so the backup might never complete.