Extracting Entries from the Binary Log

This article is relevant if the problem is on a replication slave.

Note: this text has been extracted into a separate article from Reporting bugs, see its full history there.

Sometimes a binary log event causes an error of some sort. A whole binary log file is sometimes impractical due to size or sensitivity reasons.

Step 1: Copy the binary log locally

This is just in case you don't quite extract the right information first. If the binlog expired off and you haven't got the right information, your bug report may not easily be reproducible.

sudo cp /var/lib/mysql/mysql-bin.000687 ~/
sudo chown $USER: ~/mysql-bin.000687

Step 2: Create an extract header

Binary logs have a header portion. Without the header mariadb-binlog won't be able to read it. The header also contains valuable session information

We look at the binary log to see how big the header and session information is:

mariadb-binlog --base64-output=decode-rows --verbose mysql-bin.000687 | more
/*!50530 SET @@SESSION.PSEUDO_SLAVE_MODE=1*/;
/*!40019 SET @@session.max_insert_delayed_threads=0*/;
/*!50003 SET @OLD_COMPLETION_TYPE=@@COMPLETION_TYPE,COMPLETION_TYPE=0*/;
DELIMITER /*!*/;
# at 4
#150323 22:45:58 server id 76  end_log_pos 245  Start: binlog v 4, server v 5.5.39-MariaDB-log created 150323 22:45:58
# at 245
#150323 22:45:58 server id 76  end_log_pos 328  Query   thread_id=9709067       exec_time=0     error_code=0
SET TIMESTAMP=1427116558.923924/*!*/;
SET @@session.pseudo_thread_id=9709067/*!*/;
SET @@session.foreign_key_checks=1, @@session.sql_auto_is_null=0, @@session.unique_checks=1, @@session.autocommit=1/*!*/;
SET @@session.sql_mode=0/*!*/;
SET @@session.auto_increment_increment=1, @@session.auto_increment_offset=1/*!*/;
/*!\C utf8 *//*!*/;
SET @@session.character_set_client=33,@@session.collation_connection=33,@@session.collation_server=8/*!*/;
SET @@session.time_zone='SYSTEM'/*!*/;
SET @@session.lc_time_names=0/*!*/;
SET @@session.collation_database=DEFAULT/*!*/;
BEGIN
/*!*/;
# at 328

We see that the session information ends at 328 because of the last line, so we extract to that point.

dd if=mysql-bin.000687 of=mysql-bin.000687-extract-offset-129619 bs=1 count=328

We need to find out at what offset the entry at 129619 ends and it might be useful to extract some previous entries as well.

mariadb-binlog --base64-output=decode-rows --verbose mysql-bin.000687 | grep  '^# at ' |  grep -C 10 '^# at 129619$'
# at 127602
# at 127690
# at 128201
# at 128290
# at 128378
# at 128829
# at 128918
# at 129006
# at 129459
# at 129548
# at 129619
# at 129647
# at 130070
# at 130097
# at 130168
# at 130196
# at 130738
# at 130942
# at 130969
# at 131040
# at 131244

Take a look at those entries with:

mariadb-binlog --base64-output=decode-rows --verbose --start-position 129006  --stop-position 130168  mysql-bin.000687 | more

Now let's assume we want to start at our original 129619 and finish before 130168

dd if=mysql-bin.000687 bs=1 skip=129619 count=$(( 130168 - 129619 ))  >> mysql-bin.000687-extract-offset-129619

Check the extract:

mariadb-binlog mysql-bin.000687-extract-offset-129619

Upload this to the private uploads or attach to the public bug report if nothing sensitive there.