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16.11. MySQL Cluster FAQ

This section answers questions that are often asked about MySQL Cluster.

  • What does “NDB” mean?

    This stands for “Network Database.

  • 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 are planning to implement (asynchronous) replication for Cluster in MySQL 5.1. This will include the capability to replicate both between two clusters and between a MySQL cluster and a non-Cluster MySQL server.

  • 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 16.8, “Using High-Speed Interconnects with MySQL Cluster”, for more information about SCI.

  • 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 storage 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.

  • What do the different computers do in a 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. Ideally, there will be one node per cluster host, although it is possible to run multiple nodes on a single host. There are three types of nodes, each corresponding to a specific role within the cluster. These are:

    • Management node (MGM node): Provides management services for the cluster as a whole, including startup, shutdown, backups, and configuration data for the other nodes. The management node server is implemented as the application ndb_mgmd; the management client used to control MySQL Cluster via the MGM node is ndb_mgm.

    • Data node: Stores and replicates data. Data node functionality is handled by an instance of the NDB data node process ndbd.

    • SQL node: This is simply an instance of MySQL Server (mysqld) that is built with support for the NDB Cluster storage engine and started with the --ndb-cluster option to enable the engine.

  • With which operating systems can I use Cluster?

    MySQL Cluster is officially supported on Linux, Mac OS X, and Solaris. We are working to add Cluster support for other platforms, including Windows, and our goal is eventually to offer MySQL Cluster on all platforms for which MySQL itself is supported.

    It may be possible to run Cluster processes on other operating systems. We have had reports from users who say that they have run Cluster successfully on FreeBSD. However, Cluster on any but the three platforms mentioned here should be considered alpha software (at best), cannot be guaranteed reliable in a production setting, and is not supported by MySQL AB.

  • 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.

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

    Currently, 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. This in addition to the memory required by the operating system and any applications running on the cluster computers.

    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 11.5, “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:

    • Each primary key or hash index created for an NDBCluster table requires 21–25 bytes per record. These indexes use IndexMemory.

    • Each ordered index requires 10 bytes storage per record, using DataMemory.

    • Creating a primary key or unique index also creates an ordered index, unless this index is created with USING HASH. In other words, if created without USING HASH, a primary key or unique index on a Cluster table takes up 31–35 bytes per record in MySQL 5.1.

      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. 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).

    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.

    We often see questions from users who report that, when they are trying to populate a Cluster database, the loading process terminates prematurely and an error message like this one is observed:

    ERROR 1114: The table 'my_cluster_table' is full
    

    When this occurs, 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, as 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 three computers hosting Cluster data nodes, with two of these having 3GB of RAM available to store Cluster data, and one having only 1GB RAM, then each data node can devote only 1GB to clustering.

  • Because MySQL Cluster uses TCP/IP, does that mean I can run it over the Internet, with one or more nodes in a remote location?

    It is very doubtful in any case 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.)

  • Do I have to learn a new programming or query language to use 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:

    • Creating, altering, and dropping tables

    • Inserting, updating, and deleting table data

    • Creating, changing, and dropping primary and unique indexes

    • Configuring and managing SQL nodes (MySQL servers)

  • How do I find out what an error or warning message means when using Cluster?

    There are two ways in which this can be done:

    • From within the mysql client, use SHOW ERRORS or SHOW WARNINGS immediately upon being notified of the error or warning condition. Errors and warnings also be displayed in MySQL Query Browser.

    • From a system shell prompt, use perror --ndb error_code.

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

    Yes: For tables created with the NDB storage engine, transactions are supported. In MySQL 5.1, Cluster supports only the READ COMMITTED transaction isolation level.

  • 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.)

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

    Cluster is supported in all server binaries in the 5.1 release series for operating systems on which MySQL Cluster is available (currently Linux, Mac OS X, and Solaris). 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.

    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 distibution in the MySQL 5.1 series, visit http://dev.mysql.com/downloads/mysql/5.1.html.

  • 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.)

  • Is it possible to use FULLTEXT indexes with Cluster?

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

  • 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.

  • 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.

  • Are there any limitations that I should be aware of when using Cluster?

    NDB tables in MySQL are subject to the following limitations:

    • Not all character sets and collations are supported.

    • FULLTEXT indexes and index prefixes are not supported. Only complete columns may be indexed.

    • Spatial data types are not supported. See Chapter 18, Spatial Extensions.

    • Only complete rollbacks for transactions are supported. Partial rollbacks and rollbacks to savepoints are not supported.

    • The maximum number of attributes allowed per table is 128, and attribute names cannot be any longer than 31 characters. For each table, the maximum combined length of the table and database names is 122 characters.

    • The maximum size for a table row is 8 kilobytes, not counting BLOBs. There is no set limit for the number of rows per table. Table size limits depend on a number of factors, in particular on the amount of RAM available to each data node.

    • The NDB engine does not support foreign key constraints. As with MyISAM tables, these are ignored.

    • Query caching is not supported.

    For additional information on Cluster limitations, see Section 16.9, “Known Limitations of MySQL Cluster”.

  • 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 limitation mentioned in the previous question, 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 ALTER TABLE, but requires an additional workaround. See Section 16.9, “Known Limitations of MySQL Cluster”, for details.

  • 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. 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 16.8, “Using High-Speed Interconnects with MySQL Cluster”, for more about using SCI as a transport mechanism in MySQL Cluster.

  • 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 16.4.4.4, “Defining the MySQL Cluster 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.

  • 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 18, 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 16.9, “Known Limitations of MySQL Cluster”, for more information about these issues.

  • 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 can verify the cluster is running by starting the MGM management client ndb_mgm on the machine housing the MGM node.

  • What happens to cluster data when the cluster is shut down?

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

    To shut down the cluster, enter the following command in a shell on the machine hosting the MGM node:

    shell> ndb_mgm -e shutdown
    

    This 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 16.6.2, “Commands in the Management Client”, and Section 16.3.6, “Safe Shutdown and Restart”.

  • 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.

  • Can I mix different kinds of hardware and operating systems in a 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.

  • 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, each node must 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.

  • 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.


 
 
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