OpenSearch/docs/reference/index-modules/store.asciidoc

[[index-modules-store]]
== Store

The store module allows you to control how index data is stored and accessed on disk.

[float]
[[file-system]]
=== File system storage types

There are different file system implementations or _storage types_. The best
one for the operating environment will be automatically chosen: `mmapfs` on
Windows 64bit, `simplefs` on Windows 32bit, and `default` (hybrid `niofs` and
`mmapfs`) for the rest.

This can be overridden for all indices by adding this to the
`config/elasticsearch.yml` file:

[source,yaml]
---------------------------------
index.store.type: niofs
---------------------------------

It is a _static_ setting that can be set on a per-index basis at index
creation time:

[source,js]
---------------------------------
PUT /my_index
{
  "settings": {
    "index.store.type": "niofs"
  }
}
---------------------------------

experimental[This is an expert-only setting and may be removed in the future]

The following sections lists all the different storage types supported.

[[simplefs]]`simplefs`::

The Simple FS type is a straightforward implementation of file system
storage (maps to Lucene `SimpleFsDirectory`) using a random access file.
This implementation has poor concurrent performance (multiple threads
will bottleneck). It is usually better to use the `niofs` when you need
index persistence.

[[niofs]]`niofs`::

The NIO FS type stores the shard index on the file system (maps to
Lucene `NIOFSDirectory`) using NIO. It allows multiple threads to read
from the same file concurrently. It is not recommended on Windows
because of a bug in the SUN Java implementation.

[[mmapfs]]`mmapfs`::

The MMap FS type stores the shard index on the file system (maps to
Lucene `MMapDirectory`) by mapping a file into memory (mmap). Memory
mapping uses up a portion of the virtual memory address space in your
process equal to the size of the file being mapped. Before using this
class, be sure you have allowed plenty of
<<vm-max-map-count,virtual address space>>.

[[default_fs]]`default_fs`::

The `default` type is a hybrid of NIO FS and MMapFS, which chooses the best
file system for each type of file. Currently only the Lucene term dictionary
and doc values files are memory mapped to reduce the impact on the operating
system. All other files are opened using Lucene `NIOFSDirectory`. Address
space settings (<<vm-max-map-count>>) might also apply if your term
dictionaries are large.