[[index-modules-store]] == Store The store module allows you to control how index data is stored. The index can either be stored in-memory (no persistence) or on-disk (the default). In-memory indices provide better performance at the cost of limiting the index size to the amount of available physical memory. When using a local gateway (the default), file system storage with *no* in memory storage is required to maintain index consistency. This is required since the local gateway constructs its state from the local index state of each node. Another important aspect of memory based storage is the fact that Elasticsearch supports storing the index in memory *outside of the JVM heap space* using the "Memory" (see below) storage type. It translates to the fact that there is no need for extra large JVM heaps (with their own consequences) for storing the index in memory. [float] [[store-throttling]] === Store Level Throttling The way Lucene, the IR library elasticsearch uses under the covers, works is by creating immutable segments (up to deletes) and constantly merging them (the merge policy settings allow to control how those merges happen). The merge process happens in an asynchronous manner without affecting the indexing / search speed. The problem though, especially on systems with low IO, is that the merge process can be expensive and affect search / index operation simply by the fact that the box is now taxed with more IO happening. The store module allows to have throttling configured for merges (or all) either on the node level, or on the index level. The node level throttling will make sure that out of all the shards allocated on that node, the merge process won't pass the specific setting bytes per second. It can be set by setting `indices.store.throttle.type` to `merge`, and setting `indices.store.throttle.max_bytes_per_sec` to something like `5mb`. The node level settings can be changed dynamically using the cluster update settings API. The default is set to `20mb` with type `merge`. If specific index level configuration is needed, regardless of the node level settings, it can be set as well using the `index.store.throttle.type`, and `index.store.throttle.max_bytes_per_sec`. The default value for the type is `node`, meaning it will throttle based on the node level settings and participate in the global throttling happening. Both settings can be set using the index update settings API dynamically. [float] [[file-system]] === File system storage types File system based storage is the default storage used. There are different 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 can also be set on a per-index basis at index creation time: [source,json] --------------------------------- curl -XPUT localhost:9200/my_index -d '{ "settings": { "index.store.type": "niofs" } }'; --------------------------------- The following sections lists all the different storage types supported. [float] [[simplefs]] ==== Simple FS The `simplefs` 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. [float] [[niofs]] ==== NIO FS The `niofs` 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]] [float] ==== MMap FS The `mmapfs` 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 your have plenty of virtual address space. See <> [[default_fs]] [float] ==== Hybrid MMap / NIO FS The `default` type stores the shard index on the file system depending on the file type by mapping a file into memory (mmap) or using Java NIO. 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 (<>) might also apply if your term dictionaries are large.