14.2.5.1. Support for Read-Only Media

You can now query InnoDB tables where the MySQL data directory is on read-only media, by enabling the --innodb-read-only configuration option at server startup.

How to Enable

To prepare an instance for read-only operation, make sure all the necessary information is flushed to the data files before storing it on the read-only medium. Run the server with change buffering disabled (innodb_change_buffering=0) and do a slow shutdown.

To enable read-only mode for an entire MySQL instance, specify the following configuration options at server startup:

--innodb-read-only=1
If the instance is on read-only media such as a DVD or CD, or the /var directory is not writeable by all: --pid-file=path_on_writeable_media and --event-scheduler=disabled

Usage Scenarios

This mode of operation is appropriate in situations such as:

Distributing a MySQL application, or a set of MySQL data, on a read-only storage medium such as a DVD or CD.
Multiple MySQL instances querying the same data directory simultaneously, typically in a data warehousing configuration. You might use this technique to avoid bottlenecks that can occur with a heavily loaded MySQL instance, or you might use different configuration options for the various instances to tune each one for particular kinds of queries.
Querying data that has been put into a read-only state for security or data integrity reasons, such as archived backup data.

Note

This feature is mainly intended for flexibility in distribution and deployment, rather than raw performance based on the read-only aspect. See Section 14.2.4.2.3, "Optimizations for Read-Only Transactions" for ways to tune the performance of read-only queries, which do not require making the entire server read-only.

How It Works

When the server is run in read-only mode through the --innodb-read-only option, certain InnoDB features and components are reduced or turned off entirely:

No change buffering is done, in particular no merges from the change buffer. To make sure the change buffer is empty when you prepare the instance for read-only operation, disable change buffering (innodb_change_buffering=0) and do a slow shutdown first.
There is no crash recovery phase at startup. The instance must have performed a slow shutdown before being put into the read-only state.
Because the redo log is not used in read-only operation, you can set innodb_log_file_size to the smallest size possible (1 MB) before making the instance read-only.
All background threads other than I/O read threads are turned off. As a consequence, a read-only instance cannot encounter any deadlocks.
Information about deadlocks, monitor output, and so on is not written to temporary files. As a consequence, SHOW ENGINE INNODB STATUS does not produce any output.
If the MySQL server is started with --innodb-read-only but the data directory is still on writeable media, the root user can still perform DCL operations such as GRANT and REVOKE.
Changes to configuration option settings that would normally change the the behavior of write operations, have no effect when the server is in read-only mode.
The MVCC processing to enforce isolation levels is turned off. All queries read the latest version of a record, because update and deletes are not possible.
The undo log is not used. Disable any settings for the innodb_undo_tablespaces and innodb_undo_directory configuration options.