Splunk® Enterprise

Managing Indexers and Clusters of Indexers

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System requirements and other deployment considerations

Clusters are groups of Splunk Enterprise indexers, so, for the most part, you just need to adhere to the system requirements for indexers. For detailed software and hardware requirements for indexers, read "System requirements" in the Installation Manual. The current topic notes additional requirements for clusters.

Summary of key requirements

These are the main issues to note:

  • Each cluster node (master, peer, or search head) must reside on a separate Splunk Enterprise instance.
  • Each node instance must run the same Splunk Enterprise version.
  • Each node instance must run on a separate machine or VM, and each machine must be running the same operating system.
  • All nodes must be connected through a high-speed network.

For example, to deploy a cluster consisting of three peers, one master, and one search head, you need five Splunk Enterprise instances running on five machines connected by a high-speed network. All instances must be at the same Splunk Enterprise version level (for example, 5.0.3). And all machines must be running the same operating system.

These are some additional issues to be aware of:

  • Compared to a non-clustered deployment, clusters require more storage, to accommodate the multiple copies of data.
  • Index replication, in and of itself, does not increase your licensing needs.
  • You cannot use a deployment server to distribute updates to peers.

See the remainder of this topic for details.

Required Splunk Enterprise instances

Each cluster node must reside on its own Splunk Enterprise instance.Therefore, the cluster must consist of at least (replication factor + 2) instances: a minimum of replication factor number of peer nodes, plus one master node and one or more search heads. For example, if you want to deploy a cluster with a replication factor of 3, you must set up at least five instances: three peers, one master, and one search head. To learn more about the replication factor, read "Replication factor" in this manual.

The size of your cluster depends on other factors besides the replication factor, such as the amount of data you need to index. See "Deployment overview" in this manual for details.

Note: While the master has search head capabilities, you should only use those capabilities for debugging purposes. The resources of the master must be dedicated to fulfilling its critical role of coordinating cluster activities. Under no circumstances should the master be employed as a production search head.

Splunk Enterprise version compatibility

You can implement clustering on any group of indexers, version 5.0 or above. However, all cluster components, including the search head, must be running the same version of Splunk Enterprise.

Machine requirements

Each component of the cluster (master node, peer nodes, and search head) must run on its own, separate machine or VM. Other than that, the hardware requirements are basically the same as for any Splunk Enterprise instance, as described in "Reference hardware" in the Installation Manual.

Note: The hardware storage needs of the master node are obviously lower than those specified in the "Reference hardware" topic, since the master does not index external data.

In addition, all cluster instances must be running on the same operating system.

Network requirements

All nodes of your cluster should reside on a high-speed network where each node can access every other node. This includes master node, peer nodes, and search head.

The nodes do not necessarily need to be on the same subnet, or even in the same data center (assuming you have an extremely fast connection between the data centers). You can adjust the various cluster timeout settings in server.conf. For help in configuring timeout settings, contact Splunk Professional Services.

Note: With sufficiently high-quality connections, it is possible to deploy the cluster across data centers. However, in the current version of Splunk Enterprise, the cluster is not site-aware. For example, in a scenario where you have peer nodes spread across two data centers, you cannot specify that one replicated copy of the cluster data reside on nodes in one data center and a second copy reside on nodes in a second data center. When the master determines how some set of data gets replicated across the cluster, it does not take peer location into consideration.

Storage considerations

When determining storage requirements for your clustered indexes, you need to consider the increased capacity, across the set of peer nodes, necessary to handle the multiple copies of data.

Clusters use the usual settings for managing index storage, as described in "Configure index storage".

Determine your storage requirements

It's important to ensure you have enough disk space to accommodate the volume of data your peer nodes will be processing. For a general discussion of Splunk Enterprise data volume and how to estimate your storage needs, refer to "Estimating your storage requirements" in the Installation Manual. That topic provides information on how to estimate storage for non-clustered indexers, so you need to supplement its guidelines to account for the extra copies of data that a cluster stores.

With a cluster, in addition to considering the volume of incoming data, you must consider the replication factor and search factor to arrive at your total storage requirements across the set of peer nodes. With a replication factor of 3, you are storing three copies of your data. You will need extra storage space to accommodate these copies, but you will not need three times as much storage. Replicated copies of non-searchable data are smaller than copies of searchable data, because they include only the data and not the associated index files. So, for example, if your replication factor is 3 and your search factor is 2, you will need more than two, but less than three, times the storage capacity compared to storing the same data on non-clustered indexers.

Exactly how much less storage your non-searchable copies require takes some additional investigation on your part. The index files excluded by non-searchable copies can vary greatly in size, depending on factors described in "Estimating your storage requirements".

Important: A master is not aware of the amount of storage on individual peer nodes, and therefore it does not take available storage into account when it makes decisions about which peer node should receive a particular set of replicated data. It also makes arbitrary decisions about which peer should make some set of replicated data searchable (in cases where the search factor is 2 or greater). Therefore, you must ensure that each peer node has sufficient storage not only for the data originating on that peer, but also for any replicated copies of data that might get streamed to it from other peers. You should continue to monitor storage usage throughout the life of the cluster.

Storage requirement examples

As a ballpark figure, incoming syslog data, once it has been compressed and indexed, occupies approximately 50% of its original size:

  • 15% for the rawdata file.
  • 35% for associated index files.

In practice, this estimate can vary substantially, based on the factors described in "Estimating your storage requirements" in the Installation Manual.

Assume you have 100GB of syslog data coming into Splunk Enterprise. In the case of a non-clustered indexer, that data would occupy approximately 50GB (50% of 100GB) of storage on the indexer. However, in the case of clusters, storage calculations must factor in the replication factor and search factor to arrive at total storage requirements across all the cluster peers. (As mentioned earlier, you cannot easily predict exactly how much storage will be required on any specific peer.)

Here are two examples of estimating cluster storage requirements, both assuming 100GB of incoming syslog data, resulting in 15GB for each set of rawdata and 35GB for each set of index files:

  • 3 peer nodes, with replication factor = 3; search factor = 2: This requires a total of 115GB across all peer nodes (averaging 38GB/peer), calculated as follows:
    • Total rawdata = (15GB * 3) = 45GB.
    • Total index files = (35GB * 2) = 70 GB.
  • 5 peer nodes, with replication factor = 5; search factor = 3: This requires a total of 180GB across all peer nodes (averaging 36GB/peer), calculated as follows:
    • Total rawdata = (15GB * 5) = 75GB.
    • Total index files = (35GB * 3) = 105 GB.

Storage hardware

In pre-6.0 versions of Splunk Enterprise, replicated copies of cluster buckets always resided in the colddb directory, even if they were hot or warm buckets. Starting with 6.0, hot and warm replicated copies reside in the db directory, the same as for non-replicated copies. This eliminates any need to consider faster storage for colddb for clustered indexes, compared to non-clustered indexes.

Licensing information

As with any Splunk Enterprise deployment, your licensing requirements are driven by the volume of data your indexers process. Contact your Splunk sales representative to purchase additional license volume. Refer to "How licensing works" in the Admin Manual for more information about Splunk Enterprise licensing.

There are just a few license issues that are specific to index replication:

  • All cluster members, including masters, peers, and search heads, need to be in an Enterprise license pool, even if they're not expected to index any data.
  • Cluster members must share the same licensing configuration.
  • Only incoming data counts against the license; replicated data does not.
  • You cannot use index replication with a free license.

Ports that the cluster nodes use

These ports must be available to cluster nodes:

  • On the master:
    • The management (splunkd) port (by default, 8089) must be available to all other cluster nodes.
  • On each peer:
    • The management port must be available to all other cluster nodes.
    • The replication port must be available to all other peer nodes.
    • The receiving port must be available to all forwarders sending data to that peer.
  • On each search head:
    • The management port must be available to all other nodes.
    • The http (splunkweb) port (by default, 8000) must be available to any browsers accessing data from the search head.

Deployment server and clusters

Do not use deployment server with cluster peers.

The deployment server is not supported as a means to distribute configurations or apps to cluster peers. To distribute configurations across the set of cluster peers, instead use the configuration bundle method outlined in the topic "Update common peer configurations".

For information on how to migrate app distribution from deployment server to the configuration bundle method, see "Migrate apps to a cluster".

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This documentation applies to the following versions of Splunk® Enterprise: 6.0, 6.0.1, 6.0.2, 6.0.3, 6.0.4, 6.0.5, 6.0.6, 6.0.7, 6.0.8, 6.0.9, 6.0.10, 6.0.11, 6.0.12, 6.0.13, 6.0.14, 6.0.15


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