OpenSearch/docs/reference/indices/segments.asciidoc

[[indices-segments]]
== Indices Segments

Provide low level segments information that a Lucene index (shard level)
is built with. Allows to be used to provide more information on the
state of a shard and an index, possibly optimization information, data
"wasted" on deletes, and so on.

Endpoints include segments for a specific index, several indices, or
all:

[source,js]
--------------------------------------------------
curl -XGET 'http://localhost:9200/test/_segments'
curl -XGET 'http://localhost:9200/test1,test2/_segments'
curl -XGET 'http://localhost:9200/_segments'
--------------------------------------------------

Response:

[source,js]
--------------------------------------------------
{
    ...
        "_3": {
            "generation": 3,
            "num_docs": 1121,
            "deleted_docs": 53,
            "size_in_bytes": 228288,
            "memory_in_bytes": 3211,
            "committed": true,
            "search": true,
            "version": "4.6",
            "compound": true
        }
    ...
}
--------------------------------------------------

_0::         The key of the JSON document is the name of the segment. This name
             is used to generate file names: all files starting with this
             segment name in the directory of the shard belong to this segment.

generation:: A generation number that is basically incremented when needing to
             write a new segment. The segment name is derived from this
             generation number.

num_docs::   The number of non-deleted documents that are stored in this segment.

deleted_docs:: The number of deleted documents that are stored in this segment.
             It is perfectly fine if this number is greater than 0, space is
             going to be reclaimed when this segment gets merged.

size_in_bytes:: The amount of disk space that this segment uses, in bytes.

memory_in_bytes:: Segments need to store some data into memory in order to be
             searchable efficiently. This number returns the number of bytes
             that are used for that purpose. A value of -1 indicates that
             Elasticsearch was not able to compute this number.

committed::  Whether the segment has been sync'ed on disk. Segments that are
             committed would survive a hard reboot. No need to worry in case
             of false, the data from uncommitted segments is also stored in
             the transaction log so that Elasticsearch is able to replay
             changes on the next start.

search::     Whether the segment is searchable. A value of false would most
             likely mean that the segment has been written to disk but no
             refresh occurred since then to make it searchable.

version::    The version of Lucene that has been used to write this segment.

compound::   Whether the segment is stored in a compound file. When true, this
             means that Lucene merged all files from the segment in a single
             one in order to save file descriptors.

=== Verbose mode

To add additional information that can be used for debugging, use the `verbose` flag.

NOTE: The format of additional verbose information is experimental and can change at any time.

[source,js]
--------------------------------------------------
curl -XGET 'http://localhost:9200/test/_segments?verbose=true'
--------------------------------------------------

Response:

[source,js]
--------------------------------------------------
{
    ...
        "_3": {
            ...
            "ram_tree": [
                {
                    "description": "postings [PerFieldPostings(format=1)]",
                    "size_in_bytes": 2696,
                    "children": [
                        {
                            "description": "format 'Lucene50_0' ...",
                            "size_in_bytes": 2608,
                            "children" :[ ... ]
                        },
                        ...
                    ]
                },
                ...
                ]
          
        }
    ...
}
--------------------------------------------------