# Lucene 8.2.0 misc API

Misc Tools

See: Description

Packages
Package | Description
--- | ---
org.apache.lucene.document | Misc extensions of the Document/Field API.
org.apache.lucene.index | Misc index tools and index support.
org.apache.lucene.misc | Miscellaneous index tools.
org.apache.lucene.search | Misc search implementations.
org.apache.lucene.search.similarity | Misc similarity implementations.
org.apache.lucene.store | Misc Directory implementations.
org.apache.lucene.util | Memory Tracker interface which allows defining custom collector level memory trackers
org.apache.lucene.util.fst | Misc FST classes.

## Misc Tools

The misc package has various tools for splitting/merging indices, changing norms, finding high freq terms, and others.

## NativeUnixDirectory

NOTE: This uses C++ sources (accessible via JNI), which you'll have to compile on your platform.

`NativeUnixDirectory` is a Directory implementation that bypasses the OS's buffer cache (using direct IO) for any IndexInput and IndexOutput used during merging of segments larger than a specified size (default 10 MB). This avoids evicting hot pages that are still in-use for searching, keeping search more responsive while large merges run.

See this blog post for details. Steps to build:

- `cd lucene/misc/`
- To compile NativePosixUtil.cpp -> libNativePosixUtil.so, run`ant build-native-unix`.
- `libNativePosixUtil.so` will be located in the `lucene/build/native/` folder
- Make sure libNativePosixUtil.so is on your LD_LIBRARY_PATH so java can find it (something like `export LD_LIBRARY_PATH=/path/to/dir:$LD_LIBRARY_PATH`, where /path/to/dir contains libNativePosixUtil.so)
- `ant jar` to compile the java source and put that JAR on your CLASSPATH

NativePosixUtil.cpp/java also expose access to the posix_madvise, madvise, posix_fadvise functions, which are somewhat more cross platform than O_DIRECT, however, in testing (see above link), these APIs did not seem to help prevent buffer cache eviction.

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