The role of Full Stack web developers is becoming increasingly popular. A Full Stack Web Developer is one who has a working knowledge of all components involved in website development. This includes front-end programming, back-end programming and server administration.
Server administration is a task that requires a lot of patience. Very often stressed out developers make the mistake of deleting important files. In this article I will describe my experience of restoring innodb database tables after accidentally removing innodb log files.
A server that I was in-charge of started crashing after every few hours. Initially the problem appeared to be caused by SQL injection attacks on a website. I was able to stop these attacks by configuring Fail2Ban. Even after configuring Fail2Ban, the server did not stop crashing.
I checked the running processes. The list showed two instances of MySQL server. Also the MySQL error log file showed that several database tables had crashed. Unfortunately I panicked and made the wrong decision of deleting the log files used by MySQL database server.
I had deleted the innodb1, ib_logfile0 and ib_logfile1 files. These files should not be removed as they are used by the innodb engine to store important table meta data. After removing the files all innodb database tables were inaccessible.
Fortunately the innodb_file_per_table option was enabled in MySQL server configuration. This option was enabled by default. When enabled, each innodb table uses one tablespace file with .ibd extension and a table structure file with .frm extension. innodb also uses a system table space, which is saved in the innodb1 file. I had removed the system tablespace, but fortunately, the tablespace files for each table were present.
Recovering crashed innodb tables is a common problem. The article Recover MySQL InnoDB tables without ibdata1 file, is an excellant guide on how to recover crashed innodb tables.
It was used as the main reference for this article. Other useful related articles are: MySQL InnoDB lost tables but files exist, InnoDB: Error: tablespace id in file, Troubleshooting InnoDB Data Dictionary Operations and Moving or Copying InnoDB Tables.
To recover the data, first the MySQL data directory which is /var/lib/mysql had to be backed up. Next the table schema for each innodb table had to be recovered using the mysqlfrm tool. This tool is provided by the mysql-utilities package.
The tool parses the .frm file for the innodb table and displays the CREATE TABLE statement to console. The CREATE TABLE statement can be copied to a new database. In this way the structure for all the innodb tables can be copied to a new database. Once the schema for the table is recovered, the data for the table needs to be recovered.
The data for the table is saved in a file with .ibd extension. To recover the table data, first the tablespace of the table needs to be discarded using the SQL query: ALTER TABLE table-name DISCARD TABLESPACE. Next the .ibd files for the table needs to be copied from the backup folder to the folder of the database that will contain the recovered data. Once this is done, the data for the table needs to be imported using the SQL query: ALTER TABLE table-name IMPORT TABLESPACE. Next issue the SQL query: SELECT * FROM table-name should show the table data.
The table can now be exported using mysqldump tool or PhpMyAdmin. The original database table, which is corrupted can be dropped and the new table can be imported. This needs to be done for each innodb table.
To make the task for recovering the data easier, I wrote a PHP script which recovers all innodb tables in a given database.
To use the script do the following:
Download the script using the command: wget https://gist.github.com/nadirlc/7bcf0fcc2b85e6fc15812b46525f51da
Rename the script to restore_innodb.php. To see the script options run: PHP restore_innodb.php
Run the script with the correct options. For example:
php restore_innodb.php --src-db="mysql" --backup-folder="/root/scripts/data-backup/" --mysql_data_dir="/var/lib/mysql" --log_file="/root/scripts/restore_innodb.log" --user="root" --password="root-password"
just replace the line: $src_db = “mysql”;, with the name of the database that contains the crashed tables. Also replace the line:
$backup_folder = "/root/scripts/data-backup/";
with the name of the folder that contains the backup data. The backup data should include the .frm and .idb files.
The script will replace the corrupted table with the restored table. The script may not work for all cases, because the .ibd files may be corrupt.
Innodb engine has some useful features. For example Innodb database engine supports transactions and row level locking. It is also ACID (Atomicity, Consistency, Integrity and Durability) compliant. It provides supports for foreign keys and also has faster write access than the MyISAM database engine.
However Innodb tables can easily become corrupted because of improper database server shutdown. In most cases the Innodb engine can repair corrupt tables automatically using the Innodb log files, but in some cases the Database Administrator needs to recover the data manually.