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_. By default,
Elasticsearch will pick the best implementation based on the operating
environment.

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"
  }
}
---------------------------------
// CONSOLE

WARNING: This is an expert-only setting and may be removed in the future.

The following sections lists all the different storage types supported.

`fs`::

Default file system implementation. This will pick the best implementation
depending on the operating environment, which is currently `mmapfs` on all
supported systems but is subject to change.

[[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>>.

=== Pre-loading data into the file system cache

NOTE: This is an expert setting, the details of which may change in the future.

By default, Elasticsearch completely relies on the operating system file system
cache for caching I/O operations. It is possible to set `index.store.preload`
in order to tell the operating system to load the content of hot index
files into memory upon opening. This setting accept a comma-separated list of
files extensions: all files whose extension is in the list will be pre-loaded
upon opening. This can be useful to improve search performance of an index,
especially when the host operating system is restarted, since this causes the
file system cache to be trashed. However note that this may slow down the
opening of indices, as they will only become available after data have been
loaded into physical memory.

This setting is best-effort only and may not work at all depending on the store
type and host operating system.

The `index.store.preload` is a static setting that can either be set in the
`config/elasticsearch.yml`:

[source,yaml]
---------------------------------
index.store.preload: ["nvd", "dvd"]
---------------------------------

or in the index settings at index creation time:

[source,js]
---------------------------------
PUT /my_index
{
  "settings": {
    "index.store.preload": ["nvd", "dvd"]
  }
}
---------------------------------
// CONSOLE

The default value is the empty array, which means that nothing will be loaded
into the file-system cache eagerly. For indices that are actively searched,
you might want to set it to `["nvd", "dvd"]`, which will cause norms and doc
values to be loaded eagerly into physical memory. These are the two first
extensions to look at since Elasticsearch performs random access on them.

A wildcard can be used in order to indicate that all files should be preloaded:
`index.store.preload: ["*"]`. Note however that it is generally not useful to
load all files into memory, in particular those for stored fields and term
vectors, so a better option might be to set it to
`["nvd", "dvd", "tim", "doc", "dim"]`, which will preload norms, doc values,
terms dictionaries, postings lists and points, which are the most important
parts of the index for search and aggregations.

Note that this setting can be dangerous on indices that are larger than the size
of the main memory of the host, as it would cause the filesystem cache to be
trashed upon reopens after large merges, which would make indexing and searching
_slower_.