Variable-length column support.  The NDBCLUSTER storage engine now supports variable-length column types for in-memory tables.

Previously, for example, any Cluster table having one or more VARCHAR fields which contained only relatively small values, much more memory and disk space were required when using the NDBCLUSTER storage engine than would have been the case for the same table and data using the MyISAM engine. In other words, in the case of a VARCHAR column, such a column required the same amount of storage as a CHAR column of the same size. In MySQL 5.1, this is no longer the case for in-memory tables, where storage requirements for variable-length column types such as VARCHAR and BINARY are comparable to those for these column types when used in MyISAM tables (see Section 10.5, “Data Type Storage Requirements”).

Replication with MySQL Cluster.  It is now possible to use MySQL replication with Cluster databases. For details, see Section 20.11, “MySQL Cluster Replication”.

Circular Replication.  Circular replication is supported for MySQL Cluster beginning with MySQL 5.1.18. See Section 20.11.3, “Known Issues in MySQL Cluster Replication”.

auto_increment_increment and auto_increment_offset.  The auto_increment_increment and auto_increment_offset server system variables are supported for Cluster replication beginning with MySQL 5.1.20.

Database autodiscovery and online schema changes.  Autodiscovery of databases is now supported for multiple MySQL servers accessing the same MySQL Cluster, provided that a given mysqld is already running and is connected to the cluster at the time that the database is created on a different mysqld.

What this means is that if a mysqld process first connects to the cluster after a database named db_name has been created, you should issue a CREATE SCHEMA db_name statement on the “new” MySQL server when it first accesses that MySQL Cluster. Once this has been done, the “new” mysqld should be able to detect any tables in that database tables without errors.

This also means that online schema changes in NDB tables are now possible. That is, the result of operations such as ALTER TABLE and CREATE INDEX performed on one SQL node in the cluster are now visible to the cluster's other SQL nodes without any additional action being taken.

Backup and restore between architectures.  Beginning with MySQL 5.1.10, it is possible to perform a Cluster backup and restore between different architectures. Previously — for example — you could not back up a cluster running on a big-endian platform and then restore from that backup to a cluster running on a little-endian system. (Bug#19255)

Character set directory.  Beginning with MySQL 5.1.10, it is possible to install MySQL with Cluster support to a non-default location and change the search path for font description files using either the --basedir or --character-sets-dir options. (Previously, ndbd in MySQL 5.1 searched only the default path — typically /usr/local/mysql/share/mysql/charsets — for character sets.)

In MySQL 5.1, it is no longer necessary, when running multiple management servers, to restart all the cluster's data nodes to enable the management nodes to see one another.

Length of CREATE TABLE statements.  CREATE TABLE statements may be no more than 4096 characters in length. This limitation affects MySQL 5.1.6, 5.1.7, and 5.1.8 only. (See Bug#17813)

IGNORE and REPLACE functionality.  In MySQL 5.1.7 and earlier, INSERT IGNORE, UPDATE IGNORE, and REPLACE were supported only for primary keys, but not for unique keys. It was possible to work around this issue by removing the constraint, then dropping the unique index, performing any inserts, and then adding the unique index again.

This limitation was removed for INSERT IGNORE and REPLACE in MySQL 5.1.8. (See Bug#17431.)

AUTO_INCREMENT columns.  In MySQL 5.1.10 and earlier versions, the maximum number of tables having AUTO_INCREMENT columns — including those belonging to hidden primary keys — was 2048.

This limitation was lifted in MySQL 5.1.11.

Maximum number of cluster nodes.  Prior to MySQL Cluster NDB 6.1.1, the total maximum number of nodes in a MySQL Cluster was 63, including all SQL nodes (MySQL Servers), API nodes (applications accessing the cluster other than MySQL servers), data nodes, and management servers.

Starting with MySQL Cluster NDB 6.1.1, up to 255 API nodes (including MySQL servers acting as cluster SQL nodes) are supported by a single cluster. The total number of data nodes and management nodes beginning with this version is 63, of which up to 48 can be data nodes.

Recovery of memory from deleted rows.  Beginning with MySQL Cluster NDB 6.3.7, memory can be reclaimed from an NDB table for reuse with any NDB table by employing OPTIMIZE TABLE, subject to the following limitations:

You can regulate the effects of OPTIMIZE on performance by adjusting the value of the global system variable ndb_optimization_delay, which sets the number of milliseconds to wait between batches of rows being processed by OPTIMIZE. The default value is 10 milliseconds. It is possible to set a lower value (to a minimum of 0), but not recommended. The maximum is 100000 milliseconds (that is, 100 seconds).