ralf.gebhardt asked me, what options do we loose and what is the alternative options.

1. --innobackupex --apply-log is the same as --prepare;

2. --innobackupex --incremental is the same as --incremental-dir, example:

mariabackup --innobackupex --defaults-file=$MYSQLTEST_VARDIR/my.cnf --incremental --no-timestamp --incremental-basedir=$targetdir/full $targetdir/inc;

mariabackup --innobackupex --defaults-file=$MYSQLTEST_VARDIR/my.cnf --apply-log $targetdir/full;

mariabackup --innobackupex --defaults-file=$MYSQLTEST_VARDIR/my.cnf --apply-log --incremental-dir=$targetdir/inc $targetdir/full;

can be exchanged with:

mariabackup --defaults-file=$MYSQLTEST_VARDIR/my.cnf --backup --incremental-basedir=$targetdir/full --target-dir=$targetdir/inc;

mariabackup --defaults-file=$MYSQLTEST_VARDIR/my.cnf --prepare --target-dir=$targetdir/full;

mariabackup --defaults-file=$MYSQLTEST_VARDIR/my.cnf --prepare --incremental-dir=$targetdir/inc --target-dir=$targetdir/full;

3. --no-timestamp is the default mode for mariabackup, see the above example;

4. --include and --tables are the same

5. --databases - according to the code in ibx_init():

  if (opt_ibx_databases != NULL) {

    if (is_path_separator(*opt_ibx_databases)) {

      xtrabackup_databases_file = opt_ibx_databases;

    } else {

      xtrabackup_databases = opt_ibx_databases;

    }

  }

--innobackupex --databases can contain both the list of databases/tables and a file name which contains the list of databases/tables, while just --databases can contain only the list of databases/tables, while for the file there is another option --databases_file.

6. --target_dir is omitted in innobackupex mode, just target directory path is required, what is the same as --target_dir in mariabackup, see the example above.

I would say we don't loose any functionality, all innobackupex options have alternative mariabackup options.

What do we gain by removing it? Is this mode only about remapping of command-line options or are there deeper differences?

serg This mode is only for options remapping, https://mariadb.com/kb/en/mariabackup-options/#-innobackupex. This task can be considered as a part of MDEV-23695. innobackupex was removed from xtrabackup in December 2018.

if it's only option remapping, this seems like a small and easy to maintain chunk of code. May be it'd be better to keep it for compatibility reasons?

serg It's about 1k lines of code which just duplicates the code from another .cc file, has it's own "my option" array (which duplicates mariabackup options with some small changes) with it's own "handle_options" function with it's own "get_one_option" callback. And all of this is only to copy one variables to another ones, like

 /*=====================*/                                                     

  xtrabackup_copy_back = opt_ibx_copy_back;                                     

  xtrabackup_move_back = opt_ibx_move_back;                                     

  opt_galera_info = opt_ibx_galera_info;                                        

  opt_slave_info = opt_ibx_slave_info;                                          

  opt_no_lock = opt_ibx_no_lock;                                                

  opt_safe_slave_backup = opt_ibx_safe_slave_backup;                            

  opt_rsync = opt_ibx_rsync;                                                    

  opt_force_non_empty_dirs = opt_ibx_force_non_empty_dirs;                      

  opt_noversioncheck = opt_ibx_noversioncheck;                                  

  opt_no_backup_locks = opt_ibx_no_backup_locks;                                

  opt_decompress = opt_ibx_decompress;                                          

There is no effort to maintain it at all(I think this is because there is almost no difference between innobackupex and mariabackup options, and because nobody use innobackupex, and we don't have related bug reports). And you are correct, we gain nothing in functionality too by removing it. It's just code clean-up task to get rid of copy-pasted code. And if we really need the compatibility with innobackupex, we can keep it.

If indeed "nobody use innobackupex", then we'd better remove this 1K lines of code.

Please consider adding a "timestamp" option to normal mode, so users are not forced to use

date --utc "+%F_%H-%M-%S"

to generate a sub-directory name.
Thank you.