17.5.12.1. MySQL Cluster Disk Data Objects

MySQL Cluster Disk Data storage is implemented using a number of Disk Data objects. These include the following:

Tablespaces act as containers for other Disk Data objects.
Undo log files undo information required for rolling back transactions.
One or more undo log files are assigned to a log file group, which is then assigned to a tablespace.
Data files store Disk Data table data. A data file is assigned directly to a tablespace.

Undo log files and data files are actual files in the file system of each data node; by default they are placed in ndb_node_id_fs in the DataDir specified in the MySQL Cluster config.ini file, and where node_id is the data node's node ID. It is possible to place these elsewhere by specifying either an absolute or relative path as part of the filename when creating the undo log or data file. Statements that create these files are shown later in this section.

MySQL Cluster tablespaces and log file groups are not implemented as files.

Important

Although not all Disk Data objects are implemented as files, they all share the same namespace. This means that each Disk Data object must be uniquely named (and not merely each Disk Data object of a given type). For example, you cannot have a tablespace and a log file group both named dd1.

Assuming that you have already set up a MySQL Cluster with all nodes (including management and SQL nodes), the basic steps for creating a MySQL Cluster table on disk are as follows:

Create a log file group, and assign one or more undo log files to it (an undo log file is also sometimes referred to as an undofile).

Note

Undo log files are necessary only for Disk Data tables; they are not used for NDBCLUSTER tables that are stored only in memory.
Create a tablespace; assign the log file group, as well as one or more data files, to the tablespace.
Create a Disk Data table that uses this tablespace for data storage.

Each of these tasks can be accomplished using SQL statements in the mysql client or other MySQL client application, as shown in the example that follows.

We create a log file group named lg_1 using CREATE LOGFILE GROUP. This log file group is to be made up of two undo log files, which we name undo_1.log and undo_2.log, whose initial sizes are 16 MB and 12 MB, respectively. (The default initial size for an undo log file is 128 MB.) Optionally, you can also specify a size for the log file group's undo buffer, or permit it to assume the default value of 8 MB. In this example, we set the UNDO buffer's size at 2 MB. A log file group must be created with an undo log file; so we add undo_1.log to lg_1 in this CREATE LOGFILE GROUP statement:
```
CREATE LOGFILE GROUP lg_1    ADD UNDOFILE 'undo_1.log'    INITIAL_SIZE 16M    UNDO_BUFFER_SIZE 2M    ENGINE NDBCLUSTER;
```
To add undo_2.log to the log file group, use the following ALTER LOGFILE GROUP statement:
```
ALTER LOGFILE GROUP lg_1    ADD UNDOFILE 'undo_2.log'    INITIAL_SIZE 12M    ENGINE NDBCLUSTER;
```
Some items of note:
- The .log file extension used here is not required. We use it merely to make the log files easily recognisable.
- Every CREATE LOGFILE GROUP and ALTER LOGFILE GROUP statement must include an ENGINE clause. In MySQL Cluster NDB 7.3, the only permitted values for this clause are NDBCLUSTER and NDB.
  
  Important
  
  There can exist at most one log file group in the same MySQL Cluster at any given time.
- When you add an undo log file to a log file group using ADD UNDOFILE 'filename', a file with the name filename is created in the ndb_node_id_fs directory within the DataDir of each data node in the cluster, where node_id is the node ID of the data node. Each undo log file is of the size specified in the SQL statement. For example, if a MySQL Cluster has 4 data nodes, then the ALTER LOGFILE GROUP statement just shown creates 4 undo log files, 1 each on in the data directory of each of the 4 data nodes; each of these files is named undo_2.log and each file is 12 MB in size.
- UNDO_BUFFER_SIZE is limited by the amount of system memory available.
- For more information about the CREATE LOGFILE GROUP statement, see Section 13.1.14, "CREATE LOGFILE GROUP Syntax". For more information about ALTER LOGFILE GROUP, see Section 13.1.3, "ALTER LOGFILE GROUP Syntax".
Now we can create a tablespace, which contains files to be used by MySQL Cluster Disk Data tables for storing their data. A tablespace is also associated with a particular log file group. When creating a new tablespace, you must specify the log file group which it is to use for undo logging; you must also specify a data file. You can add more data files to the tablespace after the tablespace is created; it is also possible to drop data files from a tablespace (an example of dropping data files is provided later in this section).

Assume that we wish to create a tablespace named ts_1 which uses lg_1 as its log file group. This tablespace is to contain two data files named data_1.dat and data_2.dat, whose initial sizes are 32 MB and 48 MB, respectively. (The default value for INITIAL_SIZE is 128 MB.) We can do this using two SQL statements, as shown here:
```
CREATE TABLESPACE ts_1    ADD DATAFILE 'data_1.dat'    USE LOGFILE GROUP lg_1    INITIAL_SIZE 32M    ENGINE NDBCLUSTER;ALTER TABLESPACE ts_1    ADD DATAFILE 'data_2.dat'    INITIAL_SIZE 48M    ENGINE NDBCLUSTER;
```
The CREATE TABLESPACE statement creates a tablespace ts_1 with the data file data_1.dat, and associates ts_1 with log file group lg_1. The ALTER TABLESPACE adds the second data file (data_2.dat).

Some items of note:
- As is the case with the .log file extension used in this example for undo log files, there is no special significance for the .dat file extension; it is used merely for easy recognition of data files.
- When you add a data file to a tablespace using ADD DATAFILE 'filename', a file with the name filename is created in the ndb_node_id_fs directory within the DataDir of each data node in the cluster, where node_id is the node ID of the data node. Each undo log file is of the size specified in the SQL statement. For example, if a MySQL Cluster has 4 data nodes, then the ALTER TABLESPACE statement just shown creates 4 undo log files, 1 each on in the data directory of each of the 4 data nodes; each of these files is named data_2.dat and each file is 48 MB in size.
- All CREATE TABLESPACE and ALTER TABLESPACE statements must contain an ENGINE clause; only tables using the same storage engine as the tablespace can be created in the tablespace. In MySQL MySQL Cluster NDB 7.3, the only permitted values for this clause are NDBCLUSTER and NDB.
- For more information about the CREATE TABLESPACE and ALTER TABLESPACE statements, see Section 13.1.18, "CREATE TABLESPACE Syntax", and Section 13.1.8, "ALTER TABLESPACE Syntax".
Now it is possible to create a table whose nonindexed columns are stored on disk in the tablespace ts_1:
```
CREATE TABLE dt_1 (    member_id INT UNSIGNED NOT NULL AUTO_INCREMENT PRIMARY KEY,    last_name VARCHAR(50) NOT NULL,    first_name VARCHAR(50) NOT NULL,    dob DATE NOT NULL,    joined DATE NOT NULL,    INDEX(last_name, first_name)    )    TABLESPACE ts_1 STORAGE DISK    ENGINE NDBCLUSTER;
```
The TABLESPACE ... STORAGE DISK option tells the NDBCLUSTER storage engine to use tablespace ts_1 for disk data storage.

Note

It is also possible to specify whether an individual column is stored on disk or in memory by using a STORAGE clause as part of the column's definition in a CREATE TABLE or ALTER TABLE statement. STORAGE DISK causes the column to be stored on disk, and STORAGE MEMORY causes in-memory storage to be used. See Section 13.1.17, "CREATE TABLE Syntax", for more information.

Once table ts_1 has been created as shown, you can perform INSERT, SELECT, UPDATE, and DELETE statements on it just as you would with any other MySQL table.

For table dt_1 as it has been defined here, only the dob and joined columns are stored on disk. This is because there are indexes on the id, last_name, and first_name columns, and so data belonging to these columns is stored in RAM. In MySQL Cluster NDB 7.3, only nonindexed columns can be held on disk; indexes and indexed column data continue to be stored in memory. This tradeoff between the use of indexes and conservation of RAM is something you must keep in mind as you design Disk Data tables.

Performance note. The performance of a cluster using Disk Data storage is greatly improved if Disk Data files are kept on a separate physical disk from the data node file system. This must be done for each data node in the cluster to derive any noticeable benefit.

You may use absolute and relative file system paths with ADD UNDOFILE and ADD DATAFILE. Relative paths are calculated relative to the data node's data directory. You may also use symbolic links; see Section 17.5.12.2, "Using Symbolic Links with Disk Data Objects", for more information and examples.

A log file group, a tablespace, and any Disk Data tables using these must be created in a particular order. The same is true for dropping any of these objects:

A log file group cannot be dropped as long as any tablespaces are using it.
A tablespace cannot be dropped as long as it contains any data files.
You cannot drop any data files from a tablespace as long as there remain any tables which are using the tablespace.
It is not possible to drop files created in association with a different tablespace than the one with which the files were created. (Bug #20053)

For example, to drop all the objects created so far in this section, you would use the following statements:

mysql> DROP TABLE
        dt_1;mysql> ALTER TABLESPACE
        ts_1    -> DROP DATAFILE 'data_2.dat'    -> ENGINE NDBCLUSTER;mysql> ALTER
        TABLESPACE ts_1    -> DROP DATAFILE
        'data_1.dat'    -> ENGINE NDBCLUSTER;mysql> DROP TABLESPACE ts_1    -> ENGINE
        NDBCLUSTER;mysql> DROP LOGFILE GROUP lg_1    -> ENGINE NDBCLUSTER;

These statements must be performed in the order shown, except that the two ALTER TABLESPACE ... DROP DATAFILE statements may be executed in either order.

You can obtain information about data files used by Disk Data tables by querying the FILES table in the INFORMATION_SCHEMA database. An extra "NULL row" provides additional information about undo log files. For more information and examples, see Section 20.8, "The INFORMATION_SCHEMA FILES Table".

It is also possible to view information about allocated and free disk space for each Disk Data table or table partition using the ndb_desc utility. For more information, see Section 17.4.9, "ndb_desc — Describe NDB Tables".