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A.10. MySQL 5.0 FAQ — MySQL Cluster

In the following section, we provide answers to questions that are most frequently asked about MySQL Cluster and the NDB storage engine.


Questions and Answers

26.10.1: What does “NDB” mean?

This stands for “Network Database”. NDB (also known as NDB Cluster or NDBCLUSTER) is the storage engine that enables clustering in MySQL.

26.10.2: What's the difference in using Cluster vs using replication?

In a replication setup, 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.

We have implemented (asynchronous) replication for Cluster in MySQL 5.1. This includes the capability to replicate both between two clusters, and from a MySQL cluster to a non-Cluster MySQL server. However, we do not plan to backport this functionality to MySQL 5.0.

26.10.3: Do I need to do any special networking to run Cluster? How do computers in a cluster communicate?

MySQL Cluster is intended to be used in a high-bandwidth environment, with computers connecting via TCP/IP. Its performance depends directly upon the connection speed between the cluster's computers. The minimum connectivity requirements for 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 15.9, “Using High-Speed Interconnects with MySQL Cluster”, for more information about SCI.

26.10.4: How many computers do I need to run a 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.

26.10.5: 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:

26.10.6: With which operating systems can I use Cluster?

MySQL Cluster is supported on most Unix-like operating systems, including Linux, Mac OS X, Solaris, BSD, HP-UX, AIX, and IRIX, among others, as well as Novell Netware. Cluster is not supported for Windows at this time. However, we are working to add Cluster support for other platforms, including Windows, and our goal is to offer MySQL Cluster on all platforms for which MySQL itself is supported.

For more detailed information concerning the level of support which is offered for MySQL Cluster on various operating system versions, OS distributions, and hardware platforms, please refer to

26.10.7: What are the hardware requirements for running MySQL Cluster?

Cluster should run on any platform for which NDB-enabled binaries are available. Naturally, faster CPUs and more memory will improve performance, and 64-bit CPUs will likely 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). Nodes can communicate via a standard TCP/IP network and hardware. For SCI support, special networking hardware is required.

26.10.8: How much RAM do I need? Is it possible to use disk memory at all?

In MySQL-5.0, Cluster is in-memory only. This means that all table data (including indexes) is stored in RAM. Therefore, if your data takes up 1GB of space and you want to replicate it once in the cluster, you need 2GB of memory to do so (1 GB per replica). This is in addition to the memory required by the operating system and any applications running on the cluster computers.

If a data node's memory usage exceeds what is available in RAM, then the system will attempt to use swap space up to the limit set for DataMemory. However, this will at best result in severely degraded performance, and may cause the node to be dropped due to slow response time (missed heartbeats). We do not recommend on relying on disk swapping in a production environment for this reason. In any case, once the DataMemory limit is reached, any operations requiring additional memory (such as inserts) will fail.

(We have implemented disk data storage for MySQL Cluster in MySQL 5.1, but we have no plans to add this capability in MySQL 5.0.)

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 Ч NumberOfReplicas Ч 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 11.5, “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 utility which is available in recent MySQL 5.0 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 15.8.14, “ — 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, and this primary key is created without USING HASH.

There is no easy way to determine exactly how much memory is being used for storage of Cluster indexes at any given time; however, warnings are written to the Cluster log when 80% of available DataMemory or IndexMemory is in use, and again when use reaches 85%, 90%, and so on.

26.10.9: What filesystems can I use with MySQL Cluster? What about network filesystems or network shares?

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

We do not test MySQL Cluster with FAT or VFAT filesystems on Linux. Because of this, and due to the fact that these are not very useful for any purpose other than sharing disk partitions between Linux and Windows operating systems on multi-boot computers, we do not recommend their use with 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 filesystems is not supported for MySQL Cluster. This also applies to shared storage devices such as SANs.

26.10.10: 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.

We have found that running MySQL Cluster processes inside a virtual machine can give rise to issues with timing and disk subsystems that have a strong negative impact on the operation of the cluster. The behavior of the cluster is often unpredictable in these cases.

If the issue can be reproduced outside the virtual environment, then we may be able to provide assistance. Otherwise, we cannot support it at this time.

26.10.11: I'm trying to populate a 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. In other words, 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 only 1GB to clustering.

26.10.12: 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.)

26.10.13: 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 queries and commands are required for the following operations:

Some specialized configuration parameters and files are required to set up a MySQL Cluster — see Section 15.3.4, “Configuration File”, for information about these.

A few simple commands are used in the MySQL Cluster management client for tasks such as starting and stopping cluster nodes. See Section 15.6.2, “Commands in the MySQL Cluster Management Client”.

26.10.14: 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:

26.10.15: Is MySQL Cluster transaction-safe? What isolation levels are supported?

Yes: For tables created with the NDB storage engine, transactions are supported. In MySQL 5.0, Cluster supports only the READ COMMITTED transaction isolation level.

26.10.16: What storage engines are supported by MySQL Cluster?

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

It is possible to create tables using other storage engines (such as MyISAM or InnoDB) on a MySQL server being used for clustering, but these non-NDB tables will not participate in the cluster; they are local to the individual MySQL server instance on which they are created.

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

Cluster is supported in all server binaries in the 5.0 release series for operating systems on which MySQL Cluster is available. See Section 5.2, “mysqld — The MySQL Server”. You can determine whether your server has NDB support using either the SHOW VARIABLES LIKE 'have_%' or SHOW ENGINES statement.

Linux users, please note that NDB is not included in the standard MySQL server RPMs. Beginning with MySQL 5.0.4, there are separate RPM packages for the NDB storage engine and accompanying management and other tools; see the NDB RPM Downloads section of the MySQL 5.0 Downloads page for these. (Prior to 5.0.4, you had to use the -max binaries supplied as .tar.gz archives. This is still possible, but is not required, so you can use your Linux distribution's RPM manager if you prefer.)

You can also obtain NDB support by compiling MySQL from source, but it is not necessary to do so simply to use MySQL Cluster. To download the latest binary, RPM, or source distribution in the MySQL 5.0 series, visit

26.10.18: 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.)

26.10.19: Is it possible to use FULLTEXT indexes with Cluster?

FULLTEXT indexing is not supported by the NDB storage engine in MySQL 5.0, or by any storage engine other than MyISAM. We are working to add this capability in a future release.

26.10.20: Can I run multiple nodes on a single computer?

It is possible but not advisable. One of the chief reasons to run a cluster is to provide redundancy. To enjoy 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. 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 200 MHz Pentium 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 for learning about MySQL Cluster, or for testing purposes; however, this is not supported for production use.

26.10.21: Can I add nodes to a cluster without restarting it?

Not at present. A simple restart is all that is required for adding new MGM or SQL nodes to a Cluster. When adding data nodes the process is more complex, and requires the following steps:

  1. Make a complete backup of all Cluster data.

  2. Completely shut down the cluster and all cluster node processes.

  3. Restart the cluster, using the --initial startup option.

  4. Restore all cluster data from the backup.

In a future MySQL Cluster release series, we hope to implement a “hot” reconfiguration capability for MySQL Cluster to minimize (if not eliminate) the requirement for restarting the cluster when adding new nodes. However, this is not planned for MySQL 5.0.

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

Limitations on NDB tables in MySQL 5.0 include:

For a complete listing of limitations in MySQL Cluster, see Section 15.10, “Known Limitations of MySQL Cluster”.

26.10.23: How do I import an existing MySQL database into a 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 and in Section 15.10, “Known Limitations of MySQL Cluster”, 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 using other storage engines to NDB Cluster using one or more ALTER TABLE statement, but this requires an additional workaround. See Section 15.10, “Known Limitations of MySQL Cluster”, for details.

26.10.24: How do cluster nodes communicate with one another?

Cluster nodes can communicate via any of three different protocols: 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 in MySQL 5.0. 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 15.9, “Using High-Speed Interconnects with MySQL Cluster”, for more about using SCI as a transport mechanism in MySQL Cluster.

26.10.25: What is an arbitrator?

If one or more nodes in a cluster fail, it is possible that not all cluster nodes will be able to “see” one another. In fact, it is possible that two sets of 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 nodes tries to behave as though it is the entire cluster.

When cluster nodes go down, there are two possibilities. If more than 50% of the remaining nodes can communicate with each other, we have what is sometimes called a “majority rules” situation, and this set of nodes is considered to be the cluster. The arbitrator comes into play when there is an even number of nodes: in such cases, the set of nodes to which the arbitrator belongs is considered to be the cluster, and nodes not belonging to this set are shut down.

The preceding information is somewhat simplified. A more complete explanation taking into account node groups follows:

When all nodes in at least one node group are alive, network partitioning is not an issue, because no one portion of the cluster can form a functional cluster. 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. (See Section, “Defining the Management Server”, for details.) It should also be noted that 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.

26.10.26: What data types are supported by MySQL Cluster?

MySQL Cluster supports all of the usual MySQL data types, with the exception of those associated with MySQL's spatial extensions. (See Chapter 16, Spatial Extensions.) In addition, there are some differences with regard to indexes when used with NDB tables. Note: MySQL Cluster tables (that is, tables created with ENGINE=NDBCLUSTER) have only fixed-width rows. This means that (for example) each record containing a VARCHAR(255) column will require 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. This issue is expected to be fixed in a future MySQL release series.

See Section 15.10, “Known Limitations of MySQL Cluster”, for more information about these issues.

26.10.27: 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 with the ndb_mgmd command.

  2. Start each data node with the ndbd command.

  3. Start each MySQL server (SQL node) using mysqld_safe --user=mysql &.

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 MGM management client ndb_mgm on the machine housing the MGM node and issuing the SHOW or ALL STATUS command.

To shut down a running cluster, issue the command SHUTDOWN in the MGM client. Alternatively, you may enter the following command in a system shell on the machine hosting the MGM node:

shell> ndb_mgm -e "SHUTDOWN"

(Note that the quotation marks are optional here; the SHUTDOWN command itself 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 Cluster SQL nodes can be stopped using mysqladmin shutdown.

For more information, see Section 15.6.2, “Commands in the MySQL Cluster Management Client”, and Section 15.2.6, “Safe Shutdown and Restart”.

26.10.28: What happens to cluster data when the 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.

26.10.29: Is it helpful to have more than one management node for a cluster?

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

See Section 15.3.4, “Configuration File”, for information on how to configure MySQL Cluster management nodes.

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

Yes, so long as all machines and operating systems have the same “endianness” (all big-endian or all little-endian). It is also possible to use different MySQL Cluster releases on different nodes. However, we recommend this be done only as part of a rolling upgrade procedure (see Section 15.4.1, “Performing a Rolling Restart of the Cluster”).

26.10.31: 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, running more than one cluster node of a given type per machine is not supported for production use.

26.10.32: Can I use hostnames with MySQL Cluster?

Yes, it is possible to use DNS and DHCP for cluster hosts. However, if your application requires “five nines” availability, we recommend using fixed IP addresses. Making communication between Cluster hosts dependent on services such as DNS and DHCP introduces additional points of failure, and the fewer of these, the better.

26.10.33: How do I handle MySQL users in a 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.


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