IndexConfig in SolrConfig

The <indexConfig> section of solrconfig.xml defines low-level behavior of the Lucene index writers.

By default, the settings are commented out in the sample solrconfig.xml included with Solr, which means the defaults are used. In most cases, the defaults are fine.


Writing New Segments


Once accumulated document updates exceed this much memory space (defined in megabytes), then the pending updates are flushed. This can also create new segments or trigger a merge. Using this setting is generally preferable to maxBufferedDocs. If both maxBufferedDocs and ramBufferSizeMB are set in solrconfig.xml, then a flush will occur when either limit is reached. The default is 100Mb.



Sets the number of document updates to buffer in memory before they are flushed as a new segment. This may also trigger a merge. The default Solr configuration sets to flush by RAM usage (ramBufferSizeMB).



Controls whether newly written (and not yet merged) index segments should use the Compound File Segments format. The default is false.


Merging Index Segments


Defines how merging segments is done.

The default in Solr is to use a TieredMergePolicy, which merges segments of approximately equal size, subject to an allowed number of segments per tier.

Other policies available are the LogByteSizeMergePolicy, LogDocMergePolicy, and UninvertDocValuesMergePolicy. For more information on these policies, please see the MergePolicy javadocs.

<mergePolicyFactory class="org.apache.solr.index.TieredMergePolicyFactory">
  <int name="maxMergeAtOnce">10</int>
  <int name="segmentsPerTier">10</int>

Controlling Segment Sizes: Merge Factors

The most common adjustment users make to the configuration of TieredMergePolicy (or LogByteSizeMergePolicy) are the "merge factors" to change how many segments should be merged at one time.

For TieredMergePolicy, this is controlled by setting the <int name="maxMergeAtOnce"> and <int name="segmentsPerTier"> options, while LogByteSizeMergePolicy has a single <int name="mergeFactor"> option (all of which default to 10).

To understand why these options are important, consider what happens when an update is made to an index using LogByteSizeMergePolicy: Documents are always added to the most recently opened segment. When a segment fills up, a new segment is created and subsequent updates are placed there.

If creating a new segment would cause the number of lowest-level segments to exceed the mergeFactor value, then all those segments are merged together to form a single large segment. Thus, if the merge factor is 10, each merge results in the creation of a single segment that is roughly ten times larger than each of its ten constituents. When there are 10 of these larger segments, then they in turn are merged into an even larger single segment. This process can continue indefinitely.

When using TieredMergePolicy, the process is the same, but instead of a single mergeFactor value, the segmentsPerTier setting is used as the threshold to decide if a merge should happen, and the maxMergeAtOnce setting determines how many segments should be included in the merge.

Choosing the best merge factors is generally a trade-off of indexing speed vs. searching speed. Having fewer segments in the index generally accelerates searches, because there are fewer places to look. It also can also result in fewer physical files on disk. But to keep the number of segments low, merges will occur more often, which can add load to the system and slow down updates to the index.

Conversely, keeping more segments can accelerate indexing, because merges happen less often, making an update is less likely to trigger a merge. But searches become more computationally expensive and will likely be slower, because search terms must be looked up in more index segments. Faster index updates also means shorter commit turnaround times, which means more timely search results.

Customizing Merge Policies

If the configuration options for the built-in merge policies do not fully suit your use case, you can customize them: either by creating a custom merge policy factory that you specify in your configuration, or by configuring a merge policy wrapper which uses a wrapped.prefix configuration option to control how the factory it wraps will be configured:

<mergePolicyFactory class="org.apache.solr.index.SortingMergePolicyFactory">
  <str name="sort">timestamp desc</str>
  <str name="wrapped.prefix">inner</str>
  <str name="inner.class">org.apache.solr.index.TieredMergePolicyFactory</str>
  <int name="inner.maxMergeAtOnce">10</int>
  <int name="inner.segmentsPerTier">10</int>

The example above shows Solr’s SortingMergePolicyFactory being configured to sort documents in merged segments by "timestamp desc", and wrapped around a TieredMergePolicyFactory configured to use the values maxMergeAtOnce=10 and segmentsPerTier=10 via the inner prefix defined by SortingMergePolicyFactory 's wrapped.prefix option. For more information on using SortingMergePolicyFactory, see the segmentTerminateEarly parameter.


The merge scheduler controls how merges are performed. The default ConcurrentMergeScheduler performs merges in the background using separate threads. The alternative, SerialMergeScheduler, does not perform merges with separate threads.

The ConcurrentMergeScheduler has two configurable attributes:


The maximum number of simultaneous merges that are allowed. If a merge is necessary yet we already have this many threads running, the indexing thread will block until a merge thread has completed. Note that Solr will only run the smallest maxThreadCount merges at a time.


The maximum number of simultaneous merge threads that should be running at once. This must be less than maxMergeCount.

The defaults for the above attributes are dynamically set based on whether the underlying disk drive is rotational disk or not. Refer to the Dynamic defaults for ConcurrentMergeScheduler section for more details.

Example: Dynamic defaults
<mergeScheduler class="org.apache.lucene.index.ConcurrentMergeScheduler"/>
Example: Explicit defaults
<mergeScheduler class="org.apache.lucene.index.ConcurrentMergeScheduler">
  <int name="maxMergeCount">9</int>
  <int name="maxThreadCount">4</int>


When using Solr in for Near Real Time Searching a merged segment warmer can be configured to warm the reader on the newly merged segment, before the merge commits. This is not required for near real-time search, but will reduce search latency on opening a new near real-time reader after a merge completes.

<mergedSegmentWarmer class="org.apache.lucene.index.SimpleMergedSegmentWarmer"/>

Compound File Segments

Each Lucene segment is typically comprised of a dozen or so files. Lucene can be configured to bundle all of the files for a segment into a single compound file using a file extension of .cfs; it’s an abbreviation for Compound File Segment.

CFS segments may incur a minor performance hit for various reasons, depending on the runtime environment. For example, filesystem buffers are typically associated with open file descriptors, which may limit the total cache space available to each index.

On systems where the number of open files allowed per process is limited, CFS may avoid hitting that limit. The open files limit might also be tunable for your OS with the Linux/Unix ulimit command, or something similar for other operating systems.

CFS: New Segments vs Merged Segments

To configure whether newly written segments should use CFS, see the useCompoundFile setting described above. To configure whether merged segments use CFS, review the Javadocs for your mergePolicyFactory.

Many Merge Policy implementations support noCFSRatio and maxCFSSegmentSizeMB settings with default values that prevent compound files from being used for large segments, but do use compound files for small segments.

Index Locks


The LockFactory options specify the locking implementation to use.

The set of valid lock type options depends on the DirectoryFactory you have configured. The values listed below are are supported by StandardDirectoryFactory (the default):

  • native (default) uses NativeFSLockFactory to specify native OS file locking. If a second Solr process attempts to access the directory, it will fail. Do not use when multiple Solr web applications are attempting to share a single index.

  • simple uses SimpleFSLockFactory to specify a plain file for locking.

  • single (expert) uses SingleInstanceLockFactory. Use for special situations of a read-only index directory, or when there is no possibility of more than one process trying to modify the index (even sequentially). This type will protect against multiple cores within the same JVM attempting to access the same index. WARNING! If multiple Solr instances in different JVMs modify an index, this type will not protect against index corruption.

  • hdfs uses HdfsLockFactory to support reading and writing index and transaction log files to a HDFS filesystem. See the section Running Solr on HDFS for more details on using this feature.

For more information on the nuances of each LockFactory, see



The maximum time to wait for a write lock on an IndexWriter. The default is 1000, expressed in milliseconds.


Other Indexing Settings

There are a few other parameters that may be important to configure for your implementation. These settings affect how or when updates are made to an index.


Controls how commits are retained in case of rollback. The default is SolrDeletionPolicy, which has sub-parameters for the maximum number of commits to keep (maxCommitsToKeep), the maximum number of optimized commits to keep (maxOptimizedCommitsToKeep), and the maximum age of any commit to keep (maxCommitAge), which supports DateMathParser syntax.


The InfoStream setting instructs the underlying Lucene classes to write detailed debug information from the indexing process as Solr log messages.

<deletionPolicy class="solr.SolrDeletionPolicy">
  <str name="maxCommitsToKeep">1</str>
  <str name="maxOptimizedCommitsToKeep">0</str>
  <str name="maxCommitAge">1DAY</str>
Comments on this Page

We welcome feedback on Solr documentation. However, we cannot provide application support via comments. If you need help, please send a message to the Solr User mailing list.