MySQL 9.3 Reference Manual Including MySQL NDB Cluster 9.3
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
--innodb-temp-data-file-path.
This option specifies the path, file name, and file size for
InnoDB temporary tablespace data files. The
default setting is ibtmp1:12M:autoextend,
which creates the ibtmp1 temporary
tablespace data file in the data directory. To prepare an
instance for read-only operation, set
innodb_temp_data_file_path to
a location outside of the data directory. The path must be
relative to the data directory. For example:
--innodb-temp-data-file-path=../../../tmp/ibtmp1:12M:autoextend
Enabling innodb_read_only
prevents table creation and drop operations for all storage
engines. These operations modify data dictionary tables in the
mysql system database, but those tables use the
InnoDB storage engine and cannot be modified
when innodb_read_only is enabled.
The same restriction applies to any operation that modifies data
dictionary tables, such as ANALYZE
TABLE and
ALTER TABLE
.
tbl_name
ENGINE=engine_name
In addition, other tables in the mysql system
database use the InnoDB storage engine in MySQL
9.3. Making those tables read only results in
restrictions on operations that modify them. For example,
CREATE USER,
GRANT,
REVOKE, and
INSTALL PLUGIN operations are not
permitted in read-only mode.
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 10.5.3, “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.
Most background threads are turned off. I/O read threads remain, as well as I/O write threads and a page flush coordinator thread for writes to temporary files, which are permitted in read-only mode. A buffer pool resize thread also remains active to enable online resizing of the buffer pool.
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.
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_directory
configuration option.