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5.3.12 NDB Cluster Shared-Memory Connections

Communications between NDB cluster nodes are normally handled using TCP/IP. The shared memory (SHM) transporter is distinguished by the fact that signals are transmitted by writing in memory rather than on a socket. The shared-memory transporter (SHM) can improve performance by negating up to 20% of the overhead required by a TCP connection when running an API node (usually an SQL node) and a data node together on the same host. You can enable a shared memory connection in either of the two ways listed here:

  • By setting the UseShm data node configuration parameter to 1, and setting HostName for the data node and HostName for the API node to the same value.

  • By using [shm] sections in the cluster configuration file, each containing settings for NodeId1 and NodeId2. This method is described in more detail later in this section.

Suppose a cluster is running a data node which has node ID 1 and an SQL node having node ID 51 on the same host computer at To enable an SHM connection between these two nodes, all that is necessary is to insure that the following entries are included in the cluster configuration file:


The two entries just shown are in addition to any other entries and parameter settings needed by the cluster. A more complete example is shown later in this section.

Before starting data nodes that use SHM connections, it is also necessary to make sure that the operating system on each computer hosting such a data node has sufficient memory allocated to shared memory segments. See the documentation for your operating platform for information regarding this. In setups where multiple hosts are each running a data node and an API node, it is possible to enable shared memory on all such hosts by setting UseShm in the [ndbd default] section of the configuration file. This is shown in the example later in this section.

While not strictly required, tuning for all SHM connections in the cluster can be done by setting one or more of the following parameters in the [shm default] section of the cluster configuration (config.ini) file:

  • ShmSize: Shared memory size

  • ShmSpinTime: Time in µs to spin before sleeping

  • SendBufferMemory: Size of buffer for signals sent from this node, in bytes.

  • SendSignalId: Indicates that a signal ID is included in each signal sent through the transporter.

  • Checksum: Indicates that a checksum is included in each signal sent through the transporter.

  • PreSendChecksum: Checks of the checksum are made prior to sending the signal; Checksum must also be enabled for this to work

This example shows a simple setup with SHM connections definied on multiple hosts, in an NDB Cluster using 3 computers listed here by host name, hosting the node types shown:

  1. The management server

  2. A data node and an SQL node

  3. A data node and an SQL node

In this scenario, each data node communicates with both the management server and the other data node using TCP transporters; each SQL node uses a shared memory transporter to communicate with the data nodes that is local to it, and a TCP transporter to communicate with the remote data node. A basic configuration reflecting this setup is enabled by the config.ini file whose contents are shown here:

[ndbd default]
[shm default]
[tcp default]

Parameters affecting all shared memory transporters are set in the [shm default] section; these can be overridden on a per-connection basis in one or more [shm] sections. Each such section must be associated with a given SHM connection using NodeId1 and NodeId2; the values required for these parameters are the node IDs of the two nodes connected by the transporter. You can also identify the nodes by host name using HostName1 and HostName2, but these parameters are not required.

The API nodes for which no host names are set use the TCP transporter to communicate with data nodes independent of the hosts on which they are started; the parameters and values set in the [tcp default] section of the configuration file apply to all TCP transporters in the cluster.

For optimum performance, you can define a spin time for the SHM transporter (ShmSpinTime parameter); this affects both the data node receiver thread and the poll owner (receive thread or user thread) in NDB.

  • Checksum

    This parameter is a boolean (Y/N) parameter which is disabled by default. When it is enabled, checksums for all messages are calculated before being placed in the send buffer.

    This feature prevents messages from being corrupted while waiting in the send buffer. It also serves as a check against data being corrupted during transport.

  • Group

    Determines the group proximity; a smaller value is interpreted as being closer. The default value is sufficient for most conditions.

  • HostName1

    The HostName1 and HostName2 parameters can be used to specify specific network interfaces to be used for a given SHM connection between two nodes. The values used for these parameters can be host names or IP addresses.

  • HostName2

    The HostName1 and HostName2 parameters can be used to specify specific network interfaces to be used for a given SHM connection between two nodes. The values used for these parameters can be host names or IP addresses.

  • NodeId1

    To identify a connection between two nodes it is necessary to provide node identifiers for each of them, as NodeId1 and NodeId2.

  • NodeId2

    To identify a connection between two nodes it is necessary to provide node identifiers for each of them, as NodeId1 and NodeId2.

  • NodeIdServer

    Identify the server end of a shared memory connection. By default, this is the node ID of the data node.

  • OverloadLimit

    When more than this many unsent bytes are in the send buffer, the connection is considered overloaded. See Section 5.3.14, “Configuring NDB Cluster Send Buffer Parameters”, and Section 7.14.55, “The ndbinfo transporters Table”, for more information.

  • PreSendChecksum

    If this parameter and Checksum are both enabled, perform pre-send checksum checks, and check all SHM signals between nodes for errors. Has no effect if Checksum is not also enabled.

  • SendBufferMemory

    Size (in bytes) of the shared memory buffer for signals sent from this node using a shared memory connection.

  • SendSignalId

    To retrace the path of a distributed message, it is necessary to provide each message with a unique identifier. Setting this parameter to Y causes these message IDs to be transported over the network as well. This feature is disabled by default in production builds, and enabled in -debug builds.

  • ShmKey

    When setting up shared memory segments, a node ID, expressed as an integer, is used to identify uniquely the shared memory segment to use for the communication. There is no default value. If UseShm is enabled, the shared memory key is calculated automatically by NDB.

  • ShmSize

    Each SHM connection has a shared memory segment where messages between nodes are placed by the sender and read by the reader. The size of this segment is defined by ShmSize. The default value is 4MB.

  • ShmSpinTime

    When receiving, the time to wait before sleeping, in microseconds.

  • SigNum

    This parameter was used formerly to override operating system signal numbers; in NDB 8.0, it is no longer used, and any setting for it is ignored.

Restart types.  Information about the restart types used by the parameter descriptions in this section is shown in the following table:

Table 5.15 NDB Cluster restart types

Symbol Restart Type Description
N Node The parameter can be updated using a rolling restart (see Section 7.5, “Performing a Rolling Restart of an NDB Cluster”)
S System All cluster nodes must be shut down completely, then restarted, to effect a change in this parameter
I Initial Data nodes must be restarted using the --initial option