Read server options from SHOW VARIABLES rather than my.cnf

Registered by Aleksandr Kuzminsky

Is a feature request instead not a bug,

I would suggest to read the parameters from mysql i.e. show variables if no default file is provided. as it would make the usage more simple, I have just started using the product and this is the first thought that came to my mind.

Regards
-vik
p.s.
created from the bug https://bugs.launchpad.net/percona-xtrabackup/+bug/499722

Whiteboard

KAlexey: This should also solve problems with default values. For example,
all server installations default to a hard-coded datadir value,
which depends on the package. If there is no explicit datadir
specification in my.cnf, the only way for XtraBackup to get datadir is
to ask the server (bug #936934).

Or, the default value for innodb_log_file_size got changed in MySQL
5.6. So if a 5.1/5.5 installation relies in the default value, a
5.6-based XtraBackup binary would disagree about the log configuration
and fail. (Bug #1257470 and bug #1334062).

Or, there may be a mismatch between the server specified with the
--socket option and the datadir read from my.cnf (bug #1343722).

Reading configuration parameters from my.cnf should still be
supported. It can be used for offline backup (if we decide to support
them at some point). But the default behavior should be that all server
configuration options (including those written to backup-my.cnf) should
be read from the server by innobackupex and passed to the xtrabackup
binary via the command line arguments.

(?)

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