A.10 MySQL 5.1 FAQ: MySQL Cluster

In the following section, we answer questions that are frequently asked about MySQL Cluster and the NDBCLUSTER storage engine.

Questions

Questions and Answers

A.10.1: Which versions of the MySQL software support Cluster? Do I have to compile from source?

MySQL Cluster is not supported in standard MySQL Server 5.1 releases. Instead, MySQL Cluster is provided as a separate product. Currently, the following MySQL Cluster release series are available for production use:

You should use MySQL Cluster NDB 7.2 or MySQL Cluster NDB 7.3 for any new deployments; if you are using an older version of MySQL Cluster, you should upgrade to one of these soon as possible. For an overview of improvements made in MySQL Cluster NDB 7.2, see MySQL Cluster Development in MySQL Cluster NDB 7.2; for information about improvements made in MySQL Cluster NDB 7.3, see MySQL Cluster Development in MySQL Cluster NDB 7.3.

You can determine whether your MySQL Server has NDB support using one of the statements SHOW VARIABLES LIKE 'have_%', SHOW ENGINES, or SHOW PLUGINS.

A.10.2: What happened to MySQL Cluster NDB 6.4?

Because of the number and impact of new features introduced into MySQL Cluster following the General Availability of MySQL Cluster NDB 6.3, it was decided that the following release series represented a major new release series rather than a minor one, and should be known as MySQL Cluster 7.0.

The earliest development versions of MySQL Cluster NDB 7.0 were originally designated MySQL Cluster 6.4, and the first four releases in this series were identified as MySQL Cluster NDB 6.4.0 through NDB 6.4.3. MySQL Cluster NDB 7.0.4 was the fifth MySQL Cluster NDB 7.0 release; it was the successor to MySQL Cluster NDB 6.4.3.

For more information about MySQL Cluster NDB 7.0, see Section 17.1.4.3, “MySQL Cluster Development in MySQL Cluster NDB 7.0”, and and http://dev.mysql.com/doc/relnotes/mysql-cluster/7.1/en/mysql-cluster-news-7-0.html.

A.10.3: What do NDB and NDBCLUSTER mean?

NDB stands for Network Database. NDB and NDBCLUSTER are both names for the storage engine that enables clustering support in MySQL. While our developers prefer NDB, either name is correct; both names appear in our documentation, and either name can be used in the ENGINE option of a CREATE TABLE statement for creating a MySQL Cluster table.

A.10.4: What is the difference between using MySQL Cluster vs using MySQL Replication?

In traditional MySQL replication, a master MySQL server updates one or more slaves. Transactions are committed sequentially, and a slow transaction can cause the slave to lag behind the master. This means that if the master fails, it is possible that the slave might not have recorded the last few transactions. If a transaction-safe engine such as InnoDB is being used, a transaction will either be complete on the slave or not applied at all, but replication does not guarantee that all data on the master and the slave will be consistent at all times. In MySQL Cluster, all data nodes are kept in synchrony, and a transaction committed by any one data node is committed for all data nodes. In the event of a data node failure, all remaining data nodes remain in a consistent state.

In short, whereas standard MySQL replication is asynchronous, MySQL Cluster is synchronous.

Asynchronous replication for MySQL Cluster is available in MySQL 5.1 and later. MySQL Cluster Replication (also sometimes known as geo-replication) includes the capability to replicate both between two MySQL Clusters, and from a MySQL Cluster to a non-Cluster MySQL server. See Section 17.6, “MySQL Cluster Replication”.

A.10.5: Do I need any special networking to run MySQL Cluster? How do computers in a cluster communicate?

MySQL Cluster is intended to be used in a high-bandwidth environment, with computers connecting using TCP/IP. Its performance depends directly upon the connection speed between the cluster's computers. The minimum connectivity requirements for MySQL Cluster include a typical 100-megabit Ethernet network or the equivalent. We recommend you use gigabit Ethernet whenever available.

The faster SCI protocol is also supported, but requires special hardware. See Section 17.3.5, “Using High-Speed Interconnects with MySQL Cluster”, for more information about SCI.

A.10.6: How many computers do I need to run a MySQL Cluster, and why?

A minimum of three computers is required to run a viable cluster. However, the minimum recommended number of computers in a MySQL Cluster is four: one each to run the management and SQL nodes, and two computers to serve as data nodes. The purpose of the two data nodes is to provide redundancy; the management node must run on a separate machine to guarantee continued arbitration services in the event that one of the data nodes fails.

To provide increased throughput and high availability, you should use multiple SQL nodes (MySQL Servers connected to the cluster). It is also possible (although not strictly necessary) to run multiple management servers.

A.10.7: What do the different computers do in a MySQL Cluster?

A MySQL Cluster has both a physical and logical organization, with computers being the physical elements. The logical or functional elements of a cluster are referred to as nodes, and a computer housing a cluster node is sometimes referred to as a cluster host. There are three types of nodes, each corresponding to a specific role within the cluster. These are:

A.10.8: When I run the SHOW command in the MySQL Cluster management client, I see a line of output that looks like this:

id=2    @10.100.10.32  (Version: 5.6.19-ndb-7.3.7 Nodegroup: 0, *)

What does the * mean? How is this node different from the others?

The simplest answer is, It's not something you can control, and it's nothing that you need to worry about in any case, unless you're a software engineer writing or analyzing the MySQL Cluster source code.

If you don't find that answer satisfactory, here's a longer and more technical version:

A number of mechanisms in MySQL Cluster require distributed coordination among the data nodes. These distributed algorithms and protocols include global checkpointing, DDL (schema) changes, and node restart handling. To make this coordination simpler, the data nodes elect one of their number to act as leader. (This node was once referred to as a master, but this terminology was dropped to avoid confusion with master server in MySQL Replication.) There is no user-facing mechanism for influencing this selection, which is completely automatic; the fact that it is automatic is a key part of MySQL Cluster's internal architecture.

When a node acts as the leader for any of these mechanisms, it is usually the point of coordination for the activity, and the other nodes act as followers, carrying out their parts of the activity as directed by the leader. If the node acting as leader fails, then the remaining nodes elect a new leader. Tasks in progress that were being coordinated by the old leader may either fail or be continued by the new leader, depending on the actual mechanism involved.

It is possible for some of these different mechanisms and protocols to have different leader nodes, but in general the same leader is chosen for all of them. The node indicated as the leader in the output of SHOW in the management client is known internally as the DICT manager (see The DBDICT Block, in the MySQL Cluster API Developer Guide, for more information), responsible for coordinating DDL and metadata activity.

MySQL Cluster is designed in such a way that the choice of leader has no discernable effect outside the cluster itself. For example, the current leader does not have significantly higher CPU or resource usage than the other data nodes, and failure of the leader should not have a significantly different impact on the cluster than the failure of any other data node.

A.10.9: With which operating systems can I use MySQL Cluster?

MySQL Cluster is supported on most Unix-like operating systems. Beginning with MySQL Cluster NDB 7.1.3, MySQL Cluster is also supported in production on Microsoft Windows operating systems.

For more detailed information concerning the level of support which is offered for MySQL Cluster on various operating system versions, operating system distributions, and hardware platforms, please refer to http://www.mysql.com/support/supportedplatforms/cluster.html.

A.10.10: What are the hardware requirements for running MySQL Cluster?

MySQL Cluster should run on any platform for which NDB-enabled binaries are available. For data nodes and API nodes, faster CPUs and more memory are likely to improve performance, and 64-bit CPUs are likely to be more effective than 32-bit processors. There must be sufficient memory on machines used for data nodes to hold each node's share of the database (see How much RAM do I Need? for more information). For a computer which is used only for running the MySQL Cluster management server, the requirements are minimal; a common desktop PC (or the equivalent) is generally sufficient for this task. Nodes can communicate through the standard TCP/IP network and hardware. They can also use the high-speed SCI protocol; however, special networking hardware and software are required to use SCI (see Section 17.3.5, “Using High-Speed Interconnects with MySQL Cluster”).

A.10.11: How much RAM do I need to use MySQL Cluster? Is it possible to use disk memory at all?

Formerly MySQL Cluster was in-memory only. MySQL 5.1 and later also provide the ability to store MySQL Cluster on disk. (Note that we have no plans to backport this capability to previous releases.) See Section 17.5.12, “MySQL Cluster Disk Data Tables”, for more information.

For in-memory NDB tables, you can use the following formula for obtaining a rough estimate of how much RAM is needed for each data node in the cluster:

(SizeofDatabase × NumberOfReplicas × 1.1 ) / NumberOfDataNodes

To calculate the memory requirements more exactly requires determining, for each table in the cluster database, the storage space required per row (see Section 11.7, “Data Type Storage Requirements”, for details), and multiplying this by the number of rows. You must also remember to account for any column indexes as follows:

Note that creating MySQL Cluster tables with USING HASH for all primary keys and unique indexes will generally cause table updates to run more quickly—in some cases by a much as 20 to 30 percent faster than updates on tables where USING HASH was not used in creating primary and unique keys. This is due to the fact that less memory is required (because no ordered indexes are created), and that less CPU must be utilized (because fewer indexes must be read and possibly updated). However, it also means that queries that could otherwise use range scans must be satisfied by other means, which can result in slower selects.

When calculating Cluster memory requirements, you may find useful the ndb_size.pl utility which is available in recent MySQL 5.1 releases. This Perl script connects to a current (non-Cluster) MySQL database and creates a report on how much space that database would require if it used the NDBCLUSTER storage engine. For more information, see Section 17.4.23, “ndb_size.pl — NDBCLUSTER Size Requirement Estimator”.

It is especially important to keep in mind that every MySQL Cluster table must have a primary key. The NDB storage engine creates a primary key automatically if none is defined; this primary key is created without USING HASH.

You can determine how much memory is being used for storage of MySQL Cluster data and indexes at any given time using the REPORT MEMORYUSAGE command in the ndb_mgm client; see Section 17.5.2, “Commands in the MySQL Cluster Management Client”, for more information. In addition, warnings are written to the cluster log when 80% of available DataMemory or IndexMemory is in use, and again when usage reaches 85%, 90%, and so on.

A.10.12: What file systems can I use with MySQL Cluster? What about network file systems or network shares?

Generally, any file system that is native to the host operating system should work well with MySQL Cluster. If you find that a given file system works particularly well (or not so especially well) with MySQL Cluster, we invite you to discuss your findings in the MySQL Cluster Forums.

For Windows, we recommend that you use NTFS file systems for MySQL Cluster, just as we do for standard MySQL. We do not test MySQL Cluster with FAT or VFAT file systems. Because of this, we do not recommend their use with MySQL or MySQL Cluster.

MySQL Cluster is implemented as a shared-nothing solution; the idea behind this is that the failure of a single piece of hardware should not cause the failure of multiple cluster nodes, or possibly even the failure of the cluster as a whole. For this reason, the use of network shares or network file systems is not supported for MySQL Cluster. This also applies to shared storage devices such as SANs.

A.10.13: Can I run MySQL Cluster nodes inside virtual machines (such as those created by VMWare, Parallels, or Xen)?

This is possible but not recommended for a production environment with MySQL Cluster versions prior to MySQL Cluster NDB 7.2.

For deployment in virtualized environments, you should use MySQL Cluster NDB 7.2 or later.

A.10.14: I am trying to populate a MySQL Cluster database. The loading process terminates prematurely and I get an error message like this one: ERROR 1114: The table 'my_cluster_table' is full Why is this happening?

The cause is very likely to be that your setup does not provide sufficient RAM for all table data and all indexes, including the primary key required by the NDB storage engine and automatically created in the event that the table definition does not include the definition of a primary key.

It is also worth noting that all data nodes should have the same amount of RAM, since no data node in a cluster can use more memory than the least amount available to any individual data node. For example, if there are four computers hosting Cluster data nodes, and three of these have 3GB of RAM available to store Cluster data while the remaining data node has only 1GB RAM, then each data node can devote at most 1GB to MySQL Cluster data and indexes.

In some cases it is possible to get Table is full errors in MySQL client applications even when ndb_mgm -e "ALL REPORT MEMORYUSAGE" shows significant free DataMemory. You can force NDB to create extra partitions for MySQL Cluster tables and thus have more memory available for hash indexes by using the MAX_ROWS option for CREATE TABLE. In general, setting MAX_ROWS to twice the number of rows that you expect to store in the table should be sufficient.

For similar reasons, you can also sometimes encounter problems with data node restarts on nodes that are heavily loaded with data. In MySQL Cluster NDB 7.1 and later, the addition of the MinFreePct parameter helps with this issue by reserving a portion (5% by default) of DataMemory and IndexMemory for use in restarts. This reserved memory is not available for storing NDB tables or data.

A.10.15: MySQL Cluster uses TCP/IP. Does this mean that I can run it over the Internet, with one or more nodes in remote locations?

It is very unlikely that a cluster would perform reliably under such conditions, as MySQL Cluster was designed and implemented with the assumption that it would be run under conditions guaranteeing dedicated high-speed connectivity such as that found in a LAN setting using 100 Mbps or gigabit Ethernet—preferably the latter. We neither test nor warrant its performance using anything slower than this.

Also, it is extremely important to keep in mind that communications between the nodes in a MySQL Cluster are not secure; they are neither encrypted nor safeguarded by any other protective mechanism. The most secure configuration for a cluster is in a private network behind a firewall, with no direct access to any Cluster data or management nodes from outside. (For SQL nodes, you should take the same precautions as you would with any other instance of the MySQL server.) For more information, see Section 17.5.11, “MySQL Cluster Security Issues”.

A.10.16: Do I have to learn a new programming or query language to use MySQL Cluster?

No. Although some specialized commands are used to manage and configure the cluster itself, only standard (My)SQL statements are required for the following operations:

Some specialized configuration parameters and files are required to set up a MySQL Cluster—see Section 17.3.2, “MySQL Cluster Configuration Files”, for information about these.

A few simple commands are used in the MySQL Cluster management client (ndb_mgm) for tasks such as starting and stopping cluster nodes. See Section 17.5.2, “Commands in the MySQL Cluster Management Client”.

A.10.17: What programming languages and APIs are supported by MySQL Cluster?

MySQL Cluster 5.1 and later support the same programming APIs and languages as the standard MySQL Server, including ODBC, .Net, the MySQL C API, and numerous drivers for popular scripting languages such as PHP, Perl, and Python. MySQL Cluster applications written using these APIs behave similarly to other MySQL applications; they transmit SQL statements to a MySQL Server (in the case of MySQL Cluster, an SQL node), and receive responses containing rows of data. For more information about these APIs, see Chapter 21, Connectors and APIs.

MySQL Cluster 5.1 and later also support application programming using the NDB API, which provides a low-level C++ interface to MySQL Cluster data without needing to go through a MySQL Server. See The NDB API. In addition, many NDBCLUSTER management functions are exposed by the C-language MGM API; see The MGM API, for more information.

MySQL Cluster NDB 7.1 and later also support Java application programming using ClusterJ, which supports a domain object model of data using sessions and transactions. See Java and MySQL Cluster, for more information.

A.10.18: Does MySQL Cluster include any management tools?

MySQL Cluster includes a command line client for performing basic management functions. See Section 17.4.5, “ndb_mgm — The MySQL Cluster Management Client”, and Section 17.5.2, “Commands in the MySQL Cluster Management Client”.

MySQL Cluster NDB 7.0 and later is also supported by MySQL Cluster Manager, a separate product providing an advanced command line interface that can automate many MySQL Cluster management tasks such as rolling restarts and configuration changes. For more information about MySQL Cluster Manager, see MySQL™ Cluster Manager 1.3.0 User Manual.

A.10.19: How do I find out what an error or warning message means when using MySQL Cluster?

There are two ways in which this can be done:

A.10.20: Is MySQL Cluster transaction-safe? What isolation levels are supported?

Yes. For tables created with the NDB storage engine, transactions are supported. Currently, MySQL Cluster supports only the READ COMMITTED transaction isolation level.

A.10.21: What storage engines are supported by MySQL Cluster?

Clustering with MySQL is supported only by the NDB storage engine. That is, in order for a table to be shared between nodes in a MySQL Cluster, the table must be created using ENGINE=NDB (or the equivalent option ENGINE=NDBCLUSTER).

It is possible to create tables using other storage engines (such as InnoDB or MyISAM) on a MySQL server being used with a MySQL Cluster, but since these tables do not use NDB, they do not participate in clustering; each such table is strictly local to the individual MySQL server instance on which it is created.

A.10.22: In the event of a catastrophic failure—say, for instance, the whole city loses power and my UPS fails—would I lose all my data?

All committed transactions are logged. Therefore, although it is possible that some data could be lost in the event of a catastrophe, this should be quite limited. Data loss can be further reduced by minimizing the number of operations per transaction. (It is not a good idea to perform large numbers of operations per transaction in any case.)

A.10.23: Is it possible to use FULLTEXT indexes with MySQL Cluster?

FULLTEXT indexing is currently supported only by the MyISAM storage engine. See Section 12.9, “Full-Text Search Functions”, for more information.

A.10.24: Can I run multiple nodes on a single computer?

It is possible but not always advisable. One of the chief reasons to run a cluster is to provide redundancy. To obtain the full benefits of this redundancy, each node should reside on a separate machine. If you place multiple nodes on a single machine and that machine fails, you lose all of those nodes. For this reason, if you do run multiple data nodes on a single machine, it is extremely important that they be set up in such a way that the failure of this machine does not cause the loss of all the data nodes in a given node group.

Given that MySQL Cluster can be run on commodity hardware loaded with a low-cost (or even no-cost) operating system, the expense of an extra machine or two is well worth it to safeguard mission-critical data. It also worth noting that the requirements for a cluster host running a management node are minimal. This task can be accomplished with a 300 MHz Pentium or equivalent CPU and sufficient RAM for the operating system, plus a small amount of overhead for the ndb_mgmd and ndb_mgm processes.

It is acceptable to run multiple cluster data nodes on a single host that has multiple CPUs, cores, or both. MySQL Cluster NDB 7.0 and later also provide a multi-threaded version of the data node binary intended for use on such systems. For more information, see Section 17.4.3, “ndbmtd — The MySQL Cluster Data Node Daemon (Multi-Threaded)”.

It is also possible in some cases to run data nodes and SQL nodes concurrently on the same machine; how well such an arrangement performs is dependent on a number of factors such as number of cores and CPUs as well as the amount of disk and memory available to the data node and SQL node processes, and you must take these factors into account when planning such a configuration.

A.10.25: Can I add data nodes to a MySQL Cluster without restarting it?

In MySQL Cluster NDB 7.0 and later, it is possible to add new data nodes to a running MySQL Cluster without taking it offline. For more information, see Section 17.5.13, “Adding MySQL Cluster Data Nodes Online”.

For other types of MySQL Cluster nodes, a rolling restart is all that is required (see Section 17.5.5, “Performing a Rolling Restart of a MySQL Cluster”).

(In MySQL Cluster NDB 6.3 and earlier releases, it was not possible to add new data nodes without shutting down and restarting the MySQL Cluster.)

A.10.26: Are there any limitations that I should be aware of when using MySQL Cluster?

Limitations on NDB tables in MySQL NDB 6.3, MySQL Cluster NDB 7.0, and MySQL Cluster NDB 7.1 include the following:

For a complete listing of limitations in MySQL Cluster, see Section 17.1.6, “Known Limitations of MySQL Cluster”. See also Section 17.1.6.11, “Previous MySQL Cluster Issues Resolved in MySQL 5.1, MySQL Cluster NDB 6.x, and MySQL Cluster NDB 7.x”.

A.10.27: Does MySQL Cluster support foreign keys?

MySQL Cluster NDB 7.1 and earlier MySQL Cluster releases do not support foreign key contraints, and ignore foreign keys in CREATE TABLE statements (similarly to how MyISAM treats foreign key syntax).

Foreign key support comparable to that found in the InnoDB storage engine is provided by NDB beginning with MySQL Cluster NDB 7.3. Applications requiring foreign support key should use MySQL Cluster NDB 7.3 or later.

A.10.28: How do I import an existing MySQL database into a MySQL Cluster?

You can import databases into MySQL Cluster much as you would with any other version of MySQL. Other than the limitations mentioned elsewhere in this FAQ, the only other special requirement is that any tables to be included in the cluster must use the NDB storage engine. This means that the tables must be created with ENGINE=NDB or ENGINE=NDBCLUSTER.

It is also possible to convert existing tables that use other storage engines to NDBCLUSTER using one or more ALTER TABLE statement. However, the definition of the table must be compatible with the NDBCLUSTER storage engine prior to making the conversion. In MySQL 5.1, an additional workaround is also required; see Section 17.1.6, “Known Limitations of MySQL Cluster”, for details.

A.10.29: How do MySQL Cluster nodes communicate with one another?

Cluster nodes can communicate through any of three different transport mechanisms: TCP/IP, SHM (shared memory), and SCI (Scalable Coherent Interface). Where available, SHM is used by default between nodes residing on the same cluster host; however, this is considered experimental. SCI is a high-speed (1 gigabit per second and higher), high-availability protocol used in building scalable multi-processor systems; it requires special hardware and drivers. See Section 17.3.5, “Using High-Speed Interconnects with MySQL Cluster”, for more about using SCI as a transport mechanism for MySQL Cluster.

A.10.30: What is an arbitrator?

If one or more data nodes in a cluster fail, it is possible that not all cluster data nodes will be able to see one another. In fact, it is possible that two sets of data nodes might become isolated from one another in a network partitioning, also known as a split-brain scenario. This type of situation is undesirable because each set of data nodes tries to behave as though it is the entire cluster. An arbitrator is required to decide between the competing sets of data nodes.

When all data nodes in at least one node group are alive, network partitioning is not an issue, because no single subset of the cluster can form a functional cluster on its own. The real problem arises when no single node group has all its nodes alive, in which case network partitioning (the split-brain scenario) becomes possible. Then an arbitrator is required. All cluster nodes recognize the same node as the arbitrator, which is normally the management server; however, it is possible to configure any of the MySQL Servers in the cluster to act as the arbitrator instead. The arbitrator accepts the first set of cluster nodes to contact it, and tells the remaining set to shut down. Arbitrator selection is controlled by the ArbitrationRank configuration parameter for MySQL Server and management server nodes. In MySQL Cluster NDB 7.0.7 and later, you can also use the ArbitrationRank configuration parameter to control the arbitrator selection process. For more information about these parameters, see Section 17.3.2.5, “Defining a MySQL Cluster Management Server”.

The role of arbitrator does not in and of itself impose any heavy demands upon the host so designated, and thus the arbitrator host does not need to be particularly fast or to have extra memory especially for this purpose.

A.10.31: What data types are supported by MySQL Cluster?

MySQL Cluster supports all of the usual MySQL data types, including those associated with MySQL's spatial extensions; however, the NDB storage engine does not support spatial indexes. (Spatial indexes are supported only by MyISAM; see Section 11.5, “Extensions for Spatial Data”, for more information.) In addition, there are some differences with regard to indexes when used with NDB tables.

Note

MySQL Cluster Disk Data tables (that is, tables created with TABLESPACE ... STORAGE DISK ENGINE=NDB or TABLESPACE ... STORAGE DISK ENGINE=NDBCLUSTER) have only fixed-width rows. This means that (for example) each Disk Data table record containing a VARCHAR(255) column requires space for 255 characters (as required for the character set and collation being used for the table), regardless of the actual number of characters stored therein.

See Section 17.1.6, “Known Limitations of MySQL Cluster”, for more information about these issues.

A.10.32: How do I start and stop MySQL Cluster?

It is necessary to start each node in the cluster separately, in the following order:

  1. Start the management node, using the ndb_mgmd command.

    You must include the -f or --config-file option to tell the management node where its configuration file can be found.

  2. Start each data node with the ndbd command.

    Each data node must be started with the -c or --ndb-connectstring option so that the data node knows how to connect to the management server.

  3. Start each MySQL Server (SQL node) using your preferred startup script, such as mysqld_safe.

    Each MySQL Server must be started with the --ndbcluster and --ndb-connectstring options. These options cause mysqld to enable NDBCLUSTER storage engine support and how to connect to the management server.

Each of these commands must be run from a system shell on the machine housing the affected node. (You do not have to be physically present at the machine—a remote login shell can be used for this purpose.) You can verify that the cluster is running by starting the NDB management client ndb_mgm on the machine housing the management node and issuing the SHOW or ALL STATUS command.

To shut down a running cluster, issue the command SHUTDOWN in the management client. Alternatively, you may enter the following command in a system shell:

shell> ndb_mgm -e "SHUTDOWN"

(The quotation marks in this example are optional, since there are no spaces in the command string following the -e option; in addition, the SHUTDOWN command, like other management client commands, is not case-sensitive.)

Either of these commands causes the ndb_mgm, ndb_mgm, and any ndbd processes to terminate gracefully. MySQL servers running as SQL nodes can be stopped using mysqladmin shutdown.

For more information, see Section 17.5.2, “Commands in the MySQL Cluster Management Client”, and Section 17.2.6, “Safe Shutdown and Restart of MySQL Cluster”.

A.10.33: What happens to MySQL Cluster data when the MySQL Cluster is shut down?

The data that was held in memory by the cluster's data nodes is written to disk, and is reloaded into memory the next time that the cluster is started.

A.10.34: Is it a good idea to have more than one management node for a MySQL Cluster?

It can be helpful as a fail-safe. Only one management node controls the cluster at any given time, but it is possible to configure one management node as primary, and one or more additional management nodes to take over in the event that the primary management node fails.

See Section 17.3.2, “MySQL Cluster Configuration Files”, for information on how to configure MySQL Cluster management nodes.

A.10.35: Can I mix different kinds of hardware and operating systems in one MySQL Cluster?

Yes, as long as all machines and operating systems have the same endianness (all big-endian or all little-endian).

It is also possible to use software from different MySQL Cluster releases on different nodes. However, we support this only as part of a rolling upgrade procedure (see Section 17.5.5, “Performing a Rolling Restart of a MySQL Cluster”).

A.10.36: Can I run two data nodes on a single host? Two SQL nodes?

Yes, it is possible to do this. In the case of multiple data nodes, it is advisable (but not required) for each node to use a different data directory. If you want to run multiple SQL nodes on one machine, each instance of mysqld must use a different TCP/IP port. However, in MySQL 5.1, running more than one cluster node of a given type per machine is generally not encouraged or supported for production use.

We also advise against running data nodes and SQL nodes together on the same host, since the ndbd and mysqld processes may compete for memory.

A.10.37: Can I use host names with MySQL Cluster?

Yes, it is possible to use DNS and DHCP for cluster hosts. However, if your application requires five nines availability, you should use fixed (numeric) IP addresses, since making communication between Cluster hosts dependent on services such as DNS and DHCP introduces additional potential points of failure.

A.10.38: Does MySQL Cluster support IPv6?

MySQL Cluster NDB 7.0 and later support IPv6 for connections between SQL nodes (MySQL servers). However, connections between all other types of nodes must use IPv4.

In practical terms, this means that you can use IPv6 for replication between MySQL Clusters, but connections between nodes in the same MySQL Cluster must use IPv4. For more information, see Section 17.6.3, “Known Issues in MySQL Cluster Replication”.

A.10.39: How do I handle MySQL users in a MySQL Cluster having multiple MySQL servers?

MySQL user accounts and privileges are not automatically propagated between different MySQL servers accessing the same MySQL Cluster. Therefore, you must make sure that these are copied between the SQL nodes yourself. You can do this manually, or automate the task with scripts.

Warning

Do not attempt to work around this issue by converting the MySQL system tables to use the NDBCLUSTER storage engine. Only the MyISAM storage engine is supported for these tables.

A.10.40: How do I continue to send queries in the event that one of the SQL nodes fails?

MySQL Cluster does not provide any sort of automatic failover between SQL nodes. Your application must be prepared to handlethe loss of SQL nodes and to fail over between them.

A.10.41: How do I back up and restore a MySQL Cluster?

You can use the NDB native backup and restore functionality in the MySQL Cluster management client and the ndb_restore program. See Section 17.5.3, “Online Backup of MySQL Cluster”, and Section 17.4.19, “ndb_restore — Restore a MySQL Cluster Backup”.

You can also use the traditional functionality provided for this purpose in mysqldump and the MySQL server. See Section 4.5.4, “mysqldump — A Database Backup Program”, for more information.

A.10.42: What is an angel process?

This process monitors and, if necessary, attempts to restart the data node process. If you check the list of active processes on your system after starting ndbd, you can see that there are actually 2 processes running by that name, as shown here (we omit the output from ndb_mgmd and ndbd for brevity):

shell> ./ndb_mgmd

shell> ps aux | grep ndb
me      23002  0.0  0.0 122948  3104 ?        Ssl  14:14   0:00 ./ndb_mgmd
me      23025  0.0  0.0   5284   820 pts/2    S+   14:14   0:00 grep ndb

shell> ./ndbd -c 127.0.0.1 --initial

shell> ps aux | grep ndb
me      23002  0.0  0.0 123080  3356 ?        Ssl  14:14   0:00 ./ndb_mgmd
me      23096  0.0  0.0  35876  2036 ?        Ss   14:14   0:00 ./ndbd -c 127.0.0.1 --initial
me      23097  1.0  2.4 524116 91096 ?        Sl   14:14   0:00 ./ndbd -c 127.0.0.1 --initial
me      23168  0.0  0.0   5284   812 pts/2    R+   14:15   0:00 grep ndb

The ndbd process showing 0 memory and CPU usage is the angel process. It actually does use a very small amount of each, of course. It simply checks to see if the main ndbd process (the primary data node process that actually handles the data) is running. If permitted to do so (for example, if the StopOnError configuration parameter is set to false—see Section 17.3.3.1, “MySQL Cluster Data Node Configuration Parameters”), the angel process tries to restart the primary data node process.