13.6.7.2. Forcing InnoDB Recovery

If there is database page corruption, you may want to dump your tables from the database with SELECT INTO ... OUTFILE. Usually, most of the data obtained in this way is intact. However, it is possible that the corruption might cause SELECT * FROM tbl_name statements or InnoDB background operations to crash or assert, or even cause InnoDB roll-forward recovery to crash. In such cases, you can use the innodb_force_recovery option to force the InnoDB storage engine to start up while preventing background operations from running, so that you can dump your tables. For example, you can add the following line to the [mysqld] section of your option file before restarting the server:

[mysqld]
innodb_force_recovery = 4

innodb_force_recovery is 0 by default (normal startup without forced recovery) The permissible nonzero values for innodb_force_recovery follow. A larger number includes all precautions of smaller numbers. If you can dump your tables with an option value of at most 4, then you are relatively safe that only some data on corrupt individual pages is lost. A value of 6 is more drastic because database pages are left in an obsolete state, which in turn may introduce more corruption into B-trees and other database structures.

  • 1 (SRV_FORCE_IGNORE_CORRUPT)

    Let the server run even if it detects a corrupt page. Try to make SELECT * FROM tbl_name jump over corrupt index records and pages, which helps in dumping tables.

  • 2 (SRV_FORCE_NO_BACKGROUND)

    Prevent the main thread from running. If a crash would occur during the purge operation, this recovery value prevents it.

  • 3 (SRV_FORCE_NO_TRX_UNDO)

    Do not run transaction rollbacks after recovery.

  • 4 (SRV_FORCE_NO_IBUF_MERGE)

    Prevent insert buffer merge operations. If they would cause a crash, do not do them. Do not calculate table statistics.

  • 5 (SRV_FORCE_NO_UNDO_LOG_SCAN)

    Do not look at undo logs when starting the database: InnoDB treats even incomplete transactions as committed.

  • 6 (SRV_FORCE_NO_LOG_REDO)

    Do not do the log roll-forward in connection with recovery.

    With this value, you might not be able to do queries other than a basic SELECT * FROM t, with no WHERE, ORDER BY, or other clauses. More complex queries could encounter corrupted data structures and fail.

    If corruption within the table data prevents you from dumping the entire table contents, a query with an ORDER BY primary_key DESC clause might be able to dump the portion of the table after the corrupted part.

The database must not otherwise be used with any nonzero value of innodb_force_recovery. As a safety measure, InnoDB prevents users from performing INSERT, UPDATE, or DELETE operations when innodb_force_recovery is greater than 0.

You can SELECT from tables to dump them, or DROP or CREATE tables even if forced recovery is used. If you know that a given table is causing a crash on rollback, you can drop it. You can also use this to stop a runaway rollback caused by a failing mass import or ALTER TABLE. You can kill the mysqld process and set innodb_force_recovery to 3 to bring the database up without the rollback, then DROP the table that is causing the runaway rollback.

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