Hi,

Please note that ignore_db_dirs refer to the directory name, as opposed to the database name, and the difference is important here.

From your output, it looks like you previously ran
create database `lost+found`, and not
mkdir <datadir>/lost+found.

If you look into your datadir, you'll see that the existing directory is not lost+found, it's something like lost@002bfound. It gets converted back when you run show databases.

It works the same way in 5.5.38 and 10.0, see output from 5.5.38 below.

MariaDB [test]> select @@version;

+----------------+

| @@version      |

+----------------+

| 5.5.38-MariaDB |

+----------------+

1 row in set (0.00 sec)

MariaDB [test]> select @@ignore_db_dirs;

+------------------+

| @@ignore_db_dirs |

+------------------+

| lost+found       |

+------------------+

1 row in set (0.00 sec)

MariaDB [test]> select @@datadir;

+--------------------------------------------------+

| @@datadir                                        |

+--------------------------------------------------+

| /data/releases/mariadb-5.5.38-linux-x86_64/data/ |

+--------------------------------------------------+

1 row in set (0.00 sec)

MariaDB [test]> system ls /data/releases/mariadb-5.5.38-linux-x86_64/data/

aria_log.00000001  aria_log_control  ibdata1  ib_logfile0  ib_logfile1	mysql  performance_schema  test  wheezy-64.err	wheezy-64.pid

MariaDB [test]> system mkdir /data/releases/mariadb-5.5.38-linux-x86_64/data/lost+found

MariaDB [test]> system ls /data/releases/mariadb-5.5.38-linux-x86_64/data/

aria_log.00000001  aria_log_control  ibdata1  ib_logfile0  ib_logfile1	lost+found  mysql  performance_schema  test  wheezy-64.err  wheezy-64.pid

MariaDB [test]> show databases like "lost+found";

Empty set (0.00 sec)

MariaDB [test]> create database `lost+found`;

Query OK, 1 row affected (0.00 sec)

MariaDB [test]> system ls /data/releases/mariadb-5.5.38-linux-x86_64/data/

aria_log.00000001  ibdata1	ib_logfile1	lost+found  performance_schema	wheezy-64.err

aria_log_control   ib_logfile0	lost@002bfound	mysql	    test		wheezy-64.pid

MariaDB [test]> show databases like "lost+found";

+-----------------------+

| Database (lost+found) |

+-----------------------+

| lost+found            |

+-----------------------+

1 row in set (0.00 sec)

Thank you, you were correct.
'lost@002bfound' was created by a raw data directory copy from one data dir to another.

In this case it works as expected, closing a 'Not a bug'.
Please feel free to comment if you disagree.

There seems to be a bug, though. I mounted /var/lib/mysql on a separate partition before installing
mysql-server. After installing: I have /var/lib/mysql mounted as its own ext4 filesystem.

By default, the filesystem has a directory at its root named
lost+found, which is used for cleanup during bad filesystem checks.

When i install mysql-server, it mangles this directory (which doesn't
really belong to it) into "lost@002bfound", and changes ownership on
it to mysql:mysql:

ls -l /var/lib/mysql/

rw-rr- 1 root root 0 Nov 8 11:34 debian-5.1.flag
rw-rw--- 1 mysql mysql 10485760 Nov 8 11:35 ibdata1
rw-rw--- 1 mysql mysql 5242880 Nov 8 11:35 ib_logfile0
rw-rw--- 1 mysql mysql 5242880 Nov 8 11:34 ib_logfile1
drwx------ 2 mysql mysql 4096 Nov 8 11:35 lost@002bfound
drwx------ 2 mysql root 4096 Nov 8 11:35 mysql
rw------ 1 root root 6 Nov 8 11:35 mysql_upgrade_info
0 stylus:~#

Note the wrong permission lost@002bfound directory. It also now has a
db.opt file inside it.

I'm not sure how to repair this? What should I do with the db.opt file?

I already looked at: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=608938