root@ubuntu-focal:~# mariabackup --backup --user=root --rsync

[00] 2022-07-15 13:23:14 Connecting to MySQL server host: localhost, user: root, password: not set, port: not set, socket: /run/mysqld/mysqld.sock

[00] 2022-07-15 13:23:14 Using server version 10.5.16-MariaDB-1:10.5.16+maria~focal-log

mariabackup based on MariaDB server 10.5.16-MariaDB debian-linux-gnu (x86_64)

[...]

[00] 2022-07-15 13:23:15 Starting rsync as: rsync -t . --files-from=/tmp/xtrabackup_rsyncfiles_pass1 /root/xtrabackup_backupfiles/

rsync: link_stat "/var/lib/mysql/var/lib/mysql/ib_buffer_pool" failed: No such file or directory (2)

rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1207) [sender=3.1.3]

[00] 2022-07-15 13:23:15 rsync finished successfully.

[00] 2022-07-15 13:23:15 Finished a prep copy of non-InnoDB tables and files

[00] 2022-07-15 13:23:15 Acquiring BACKUP LOCKS...

[00] 2022-07-15 13:23:15 Starting to backup non-InnoDB tables and files

[00] 2022-07-15 13:23:15 Starting rsync as: rsync -t . --files-from=/tmp/xtrabackup_rsyncfiles_pass2 /root/xtrabackup_backupfiles/

rsync: link_stat "/var/lib/mysql/var/lib/mysql/ib_buffer_pool" failed: No such file or directory (2)

rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1207) [sender=3.1.3]

[00] 2022-07-15 13:23:15 Error: rsync failed with error code 1

mariabackup: Stopping log copying thread.[00] 2022-07-15 13:23:15 >> log scanned up to (55832)

During normal operation, Mariabackup transfers local

non-InnoDB files using a separate call to cp for each file. 

Using this option, you can optimize this process by 

performing this transfer with rsync, instead.