ndbmtd is a multi-threaded version of ndbd, the process that is used to handle all the data in tables using the
NDBCLUSTERstorage engine. ndbmtd is intended for use on host computers having multiple CPU cores. Except where otherwise noted, ndbmtd functions in the same way as ndbd; therefore, in this section, we concentrate on the ways in which ndbmtd differs from ndbd, and you should consult Section 17.4.2, “ndbd — The MySQL Cluster Data Node Daemon”, for additional information about running MySQL Cluster data nodes that apply to both the single-threaded and multi-threaded versions of the data node process.
Command-line options and configuration parameters used with ndbd also apply to ndbmtd. For more information about these options and parameters, see Section 17.4.2, “ndbd — The MySQL Cluster Data Node Daemon”, and Section 22.214.171.124, “Defining MySQL Cluster Data Nodes”, respectively.
ndbmtd is also file system-compatible with ndbd. In other words, a data node running ndbd can be stopped, the binary replaced with ndbmtd, and then restarted without any loss of data. (However, when doing this, you must make sure that
MaxNoOfExecutionThreadsis set to an apppriate value before restarting the node if you wish for ndbmtd to run in multi-threaded fashion.) Similarly, an ndbmtd binary can be replaced with ndbd simply by stopping the node and then starting ndbd in place of the multi-threaded binary. It is not necessary when switching between the two to start the data node binary using
Prior to MySQL Cluster NDB 7.0.6, there were known issues when using ndbmtd with MySQL Cluster Disk Data tables. If you wish to use multi-threaded data nodes with disk-based
NDBtables, you should ensure that you are running MySQL Cluster NDB 7.0.6 or later. (Bug#41915, Bug#44915)
Using ndbmtd differs from using ndbd in two key respects:
You must set an appropriate value for the
MaxNoOfExecutionThreadsconfiguration parameter in the
config.inifile. If you do not do so, ndbmtd runs in single-threaded mode—that is, it behaves like ndbd.
Trace files are generated by critical errors in ndbmtd processes in a somewhat different fashion from how these are generated by ndbd failures.
These differences are discussed in more detail in the next few paragraphs.
Number of execution threads. The
MaxNoOfExecutionThreadsconfiguration parameter is used to determine the number of local query handler (LQH) threads spawned by ndbmtd. Although this parameter is set in
[ndbd default]sections of the
config.inifile, it is exclusive to ndbmtd and does not apply to ndbd.
This parameter takes an integer value from 2 to 8 inclusive. Generally, you should set this parameter equal to the number of CPU cores on the data node host, as shown in the following table:
Number of Cores Recommended
2 2 4 4 8 or more 8
(It is possible to set this parameter to other values within the permitted range, but these are automatically rounded as shown in the Value Used column of the next table in this section.)
The multi-threaded data node process always spawns at least 4 threads:
1 local query handler (LQH) thread
1 transaction coordinator (TC) thread
1 transporter thread
1 subscription manager (SUMA) thread
Setting this parameter to a value between 4 and 8 inclusive causes additional LQH threads to be used by ndbmtd (up to a maximum of 4 LQH threads), as shown in the following table:
Value Used Number of LQH Threads Used 3 2 1 5 or 6 4 2 7 8 4
Setting this parameter outside the permitted range of values causes the management server to abort on startup with the error Error line
number: Illegal value
valuefor parameter MaxNoOfExecutionThreads.
In MySQL Cluster NDB 6.4.0, it is not possible to set
MaxNoOfExecutionThreadsto 2. You can safely use the value 3 instead (it is treated as 2 internally). This issue is resolved in MySQL Cluster NDB 6.4.1.
In MySQL Cluster NDB 6.4.0 through 6.4.3, the default value for this parameter was undefined, although the default behavior for ndbmtd was to use 1 LQH thread, as though
MaxNoOfExecutionThreadshad been set to 2. Beginning with MySQL Cluster NDB 7.0.4, this parameter has an explcit default value of 2, thus guaranteeing this default behavior.
In MySQL Cluster NDB 7.0, it is not possible to cause ndbmtd to use more than 1 TC thread, although we plan to introduce this capability in a future MySQL Cluster release series.
Like ndbd, ndbmtd generates a set of log files which are placed in the directory specified by
config.iniconfiguration file. Except for trace files, these are generated in the same way and have the same names as those generated by ndbd.
In the event of a critical error, ndbmtd generates trace files describing what happened just prior to the error' occurrence. These files, which can be found in the data node's
DataDir, are useful for analysis of problems by the MySQL Cluster Development and Support teams. One trace file is generated for each ndbmtd thread. The names of these files follow the pattern
node_idis the data node's unique node ID in the cluster,
trace_idis a trace sequence number, and
thread_idis the thread ID. For example, in the event of the failure of an ndbmtd process running as a MySQL Cluster data node having the node ID 3 and with
MaxNoOfExecutionThreadsequal to 4, four trace files are generated in the data node's data directory; if the is the first time this node has failed, then these files are named
ndb_3_trace.log.1_t4. Internally, these trace files follow the same format as ndbd trace files.
The ndbd exit codes and messages that are generated when a data node process shuts down prematurely are also used by ndbmtd. See
ndbdError Messages, for a listing of these.
It is possible to use ndbd and ndbmtd concurrently on different data nodes in the same MySQL Cluster. However, such configurations have not been tested extensively; thus, we cannot not recommend doing so in a production setting at this time.