25.6.11.1 NDB Cluster Disk Data Objects (original) (raw)

25.6.11.1 NDB Cluster Disk Data Objects

NDB Cluster Disk Data storage is implemented using the following objects:

Undo log files and data files are actual files in the file system of each data node; by default they are placed inndb_ _`nodeid`__fs in the DataDir specified in the NDB Cluster config.ini file, and where_nodeid_ 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.

Undo log files are used only by Disk Data tables, and are not needed or used by NDB tables that are stored in memory only.

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

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 an NDB Cluster with all nodes (including management and SQL nodes), the basic steps for creating an NDB Cluster table on disk are as follows:

  1. 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).
  2. Create a tablespace; assign the log file group, as well as one or more data files, to the tablespace.
  3. 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.

  1. 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 andundo_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 addundo_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:

  1. Now we can create a tablespace—an abstract container for files used by Disk Data tables to store data. A tablespace is associated with a particular log file group; when creating a new tablespace, you must specify the log file group it uses for undo logging. You must also specify at least one 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 (see example later in this section).
    Assume that we wish to create a tablespace namedts_1 which uses lg_1 as its log file group. We want the tablespace to contain two data files, named data_1.dat anddata_2.dat, whose initial sizes are 32 MB and 48 MB, respectively. (The default value forINITIAL_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;

The CREATE TABLESPACE statement creates a tablespace ts_1 with the data file data_1.dat, and associates ts_1 with log file grouplg_1. The ALTER TABLESPACE adds the second data file (data_2.dat).
Some items of note:

  1. Now it is possible to create a table whose unindexed columns are stored on disk using files in tablespacets_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;

TABLESPACE ts_1 STORAGE DISK tells theNDB storage engine to use tablespace ts_1 for data storage on disk.
Once table ts_1 has been created as shown, you can performINSERT,SELECT,UPDATE, andDELETE statements on it just as you would with any other MySQL table.
It is also possible to specify whether an individual column is stored on disk or in memory by using aSTORAGE 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. SeeSection 15.1.20, “CREATE TABLE Statement”, for more information.

You can obtain information about the NDB disk data files and undo log files just created by querying theFILES table in theINFORMATION_SCHEMA database, as shown here:

mysql> SELECT
              FILE_NAME AS File, FILE_TYPE AS Type,
              TABLESPACE_NAME AS Tablespace, TABLE_NAME AS Name,
              LOGFILE_GROUP_NAME AS 'File group',
              FREE_EXTENTS AS Free, TOTAL_EXTENTS AS Total
          FROM INFORMATION_SCHEMA.FILES
          WHERE ENGINE='ndbcluster';
+--------------+----------+------------+------+------------+------+---------+
| File         | Type     | Tablespace | Name | File group | Free | Total   |
+--------------+----------+------------+------+------------+------+---------+
| ./undo_1.log | UNDO LOG | lg_1       | NULL | lg_1       |    0 | 4194304 |
| ./undo_2.log | UNDO LOG | lg_1       | NULL | lg_1       |    0 | 3145728 |
| ./data_1.dat | DATAFILE | ts_1       | NULL | lg_1       |   32 |      32 |
| ./data_2.dat | DATAFILE | ts_1       | NULL | lg_1       |   48 |      48 |
+--------------+----------+------------+------+------------+------+---------+
4 rows in set (0.00 sec)

For more information and examples, seeSection 28.3.15, “The INFORMATION_SCHEMA FILES Table”.

Indexing of columns implicitly stored on disk. For table dt_1 as defined in the example just shown, only the dob andjoined columns are stored on disk. This is because there are indexes on the id,last_name, andfirst_name columns, and so data belonging to these columns is stored in RAM. 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.

You cannot add an index to a column that has been explicitly declared STORAGE DISK, without first changing its storage type to MEMORY; any attempt to do so fails with an error. A column which_implicitly_ uses disk storage can be indexed; when this is done, the column's storage type is changed to MEMORY automatically. By“implicitly”, we mean a column whose storage type is not declared, but which is which inherited from the parent table. In the following CREATE TABLE statement (using the tablespace ts_1 defined previously), columnsc2 and c3 use disk storage implicitly:

mysql> CREATE TABLE ti (
    ->     c1 INT PRIMARY KEY,
    ->     c2 INT,
    ->     c3 INT,
    ->     c4 INT
    -> )
    ->     STORAGE DISK
    ->     TABLESPACE ts_1
    ->     ENGINE NDBCLUSTER;
Query OK, 0 rows affected (1.31 sec)

Because c2, c3, andc4 are themselves not declared withSTORAGE DISK, it is possible to index them. Here, we add indexes to c2 andc3, using, respectively, CREATE INDEX and ALTER TABLE:

mysql> CREATE INDEX i1 ON ti(c2);
Query OK, 0 rows affected (2.72 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> ALTER TABLE ti ADD INDEX i2(c3);
Query OK, 0 rows affected (0.92 sec)
Records: 0  Duplicates: 0  Warnings: 0

SHOW CREATE TABLE confirms that the indexes were added.

mysql> SHOW CREATE TABLE ti\G
*************************** 1. row ***************************
       Table: ti
Create Table: CREATE TABLE `ti` (
  `c1` int(11) NOT NULL,
  `c2` int(11) DEFAULT NULL,
  `c3` int(11) DEFAULT NULL,
  `c4` int(11) DEFAULT NULL,
  PRIMARY KEY (`c1`),
  KEY `i1` (`c2`),
  KEY `i2` (`c3`)
) /*!50100 TABLESPACE `ts_1` STORAGE DISK */ ENGINE=ndbcluster DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci
1 row in set (0.00 sec)

You can see using ndb_desc that the indexed columns (emphasized text) now use in-memory rather than on-disk storage:

$> ./ndb_desc -d test t1
-- t1 --
Version: 33554433
Fragment type: HashMapPartition
K Value: 6
Min load factor: 78
Max load factor: 80
Temporary table: no
Number of attributes: 4
Number of primary keys: 1
Length of frm data: 317
Max Rows: 0
Row Checksum: 1
Row GCI: 1
SingleUserMode: 0
ForceVarPart: 1
PartitionCount: 4
FragmentCount: 4
PartitionBalance: FOR_RP_BY_LDM
ExtraRowGciBits: 0
ExtraRowAuthorBits: 0
TableStatus: Retrieved
Table options:
HashMap: DEFAULT-HASHMAP-3840-4
-- Attributes --
c1 Int PRIMARY KEY DISTRIBUTION KEY AT=FIXED ST=MEMORY
c2 Int NULL AT=FIXED ST=MEMORY
c3 Int NULL AT=FIXED ST=MEMORY
c4 Int NULL AT=FIXED ST=DISK
-- Indexes --
PRIMARY KEY(c1) - UniqueHashIndex
i2(c3) - OrderedIndex
PRIMARY(c1) - OrderedIndex
i1(c2) - OrderedIndex

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 can use absolute and relative file system paths withADD UNDOFILE and ADD DATAFILE; relative paths are calculated with respect to the data node's data directory.

A log file group, a tablespace, and any Disk Data tables using these must be created in a particular order. This is also true for dropping these objects, subject to the following constraints:

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

mysql> DROP TABLE dt_1;

mysql> ALTER TABLESPACE ts_1
    -> DROP DATAFILE 'data_2.dat';

mysql> ALTER TABLESPACE ts_1
    -> DROP DATAFILE 'data_1.dat';

mysql> DROP TABLESPACE ts_1;

mysql> DROP LOGFILE GROUP lg_1;

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

Note

Older versions of NDB Cluster used anENGINE clause with ALTER TABLESPACE ... DROP DATAFILE and DROP TABLESPACE. In NDB 8.4 and later, it is no longer supported with either of these statements.