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Swapping

Overview

When using a pure in-memory storage, it is possible that the size of data loaded into a node exceeds the physical RAM size, leading to out of memory errors (OOMEs). If you do not want to use the native persistence or an external storage, you can enable swapping, in which case the in-memory data is moved to the swap space located on disk. Please note that Ignite does not provide its own implementation of swap space. Instead, it takes advantage of the swapping functionality provided by the operating system (OS).

When swap space is enabled, Ignite stores data in memory-mapped files (MMF) whose content is swapped to disk by the OS according to the current RAM consumption; however, in that scenario the data access time is longer. Moreover, there are no data durability guarantees. Which means that the data from the swap space is available only as long as the node is alive. Once the node where the swap space exists shuts down, the data is lost. Therefore, you should use swap space as an extension to RAM only to give yourself enough time to add more nodes to the cluster in order to re-distribute data and avoid OOMEs which might happen if the cluster is not scaled in time.

Caution

Since swap space is located on disk, it should not be considered as a replacement for native persistence. Data from the swap space is available as long as the node is active. Once the node shuts down, the data is lost. To ensure that data is always available, you should either enable native persistence or use an external storage.

Enabling Swapping

Data Region maxSize defines the total maxSize of the region. You will get out of memory errors if your data size exceeds maxSize and neither native persistence nor an external database is used. To avoid this situation with the swapping capabilities, you need to:

Set maxSize to a value that is bigger than the total RAM size. In this case, the OS takes care of the swapping.
Enable swapping in the data region configuration, as shown below.

<bean class="org.apache.ignite.configuration.IgniteConfiguration">
    <!-- Durable memory configuration. -->
    <property name="dataStorageConfiguration">
        <bean class="org.apache.ignite.configuration.DataStorageConfiguration">
            <property name="dataRegionConfigurations">
                <list>
                    <!--
                    Defining a data region that will consume up to 500 MB of RAM
                    with swap enabled.
                    -->
                    <bean class="org.apache.ignite.configuration.DataRegionConfiguration">
                        <!-- Custom region name. -->
                        <property name="name" value="500MB_Region"/>
                        <!-- 100 MB initial size. -->
                        <property name="initialSize" value="#{100L * 1024 * 1024}"/>
                        <!-- Setting region max size equal to physical RAM size(5 GB). -->
                        <property name="maxSize" value="#{5L * 1024 * 1024 * 1024}"/>
                        <!-- Enabling swap space for the region. -->
                        <property name="swapPath" value="/path/to/some/directory"/>
                    </bean>
                </list>
            </property>
        </bean>
    </property>
    <!-- Other configurations. -->
</bean>

// Node configuration.
IgniteConfiguration cfg = new IgniteConfiguration();

// Durable Memory configuration.
DataStorageConfiguration storageCfg = new DataStorageConfiguration();

// Creating a new data region.
DataRegionConfiguration regionCfg = new DataRegionConfiguration();

// Region name.
regionCfg.setName("500MB_Region");

// Setting initial RAM size.
regionCfg.setInitialSize(100L * 1024 * 1024);

// Setting region max size equal to physical RAM size(5 GB)
regionCfg.setMaxSize(5L * 1024 * 1024 * 1024);

// Enable swap space.
regionCfg.setSwapPath("/path/to/some/directory");

// Setting the data region configuration.
storageCfg.setDataRegionConfigurations(regionCfg);

// Applying the new configuration.
cfg.setDataStorageConfiguration(storageCfg);

var cfg = new IgniteConfiguration
{
    DataStorageConfiguration = new DataStorageConfiguration
    {
        DataRegionConfigurations = new[]
        {
            new DataRegionConfiguration
            {
                Name = "5GB_Region",
                InitialSize = 100L * 1024 * 1024,
                MaxSize = 5L * 1024 * 1024 * 1024,
                SwapPath = "/path/to/some/directory"
            }
        }
    }
};

This API is not presently available for C++. You can use XML configuration.

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