Although innobackupex 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 innobackupex reads and writes InnoDB data. You can do this with the --throttle option.
This option is passed directly to xtrabackup binary and only limits the operations on the logs and files of InnoDB tables. It doesn’t have an effect on reading or writing files from tables with other storage engine.
One way of checking the current I/O operations at a system is with iostat command. See Throttling Backups for details of how throttling works.
The --throttle option is somehow similar to the --sleep option in mysqlbackup and should be used instead of it, as --sleep will be ignored.