MySQL 5.6 Reference Manual Including MySQL NDB Cluster 7.3-7.4 Reference Guide
You can query InnoDB
tables where the MySQL
data directory is on read-only media by enabling the
--innodb-read-only
configuration
option at server startup.
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:
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=
and
path_on_writeable_media
--event-scheduler=disabled
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.
This feature is mainly intended for flexibility in distribution and deployment, rather than raw performance based on the read-only aspect. See Section 8.5.3, “Optimizing InnoDB Read-Only Transactions” for ways to tune the performance of read-only queries, which do not require making the entire server read-only.
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 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.