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Indices Segments

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Indices Segments

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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:

GET /test/_segments

For several indices:

GET /test1,test2/_segments

Or for all indices:

GET /_segments

Response:

{
  "_shards": ...
  "indices": {
    "test": {
      "shards": {
        "0": [
          {
            "routing": {
              "state": "STARTED",
              "primary": true,
              "node": "zDC_RorJQCao9xf9pg3Fvw"
            },
            "num_committed_segments": 0,
            "num_search_segments": 1,
            "segments": {
              "_0": {
                "generation": 0,
                "num_docs": 1,
                "deleted_docs": 0,
                "size_in_bytes": 3800,
                "memory_in_bytes": 1410,
                "committed": false,
                "search": true,
                "version": "7.0.0",
                "compound": true,
                "attributes": {
                }
              }
            }
          }
        ]
      }
    }
  }
}

_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.
attributes: Contains information about whether high compression was enabled

Verbose mode

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

The format of the additional detail information is labelled as experimental in Lucene and it may change in the future.

GET /test/_segments?verbose=true

Response:

{
    ...
        "_0": {
            ...
            "ram_tree": [
                {
                    "description": "postings [PerFieldPostings(format=1)]",
                    "size_in_bytes": 2696,
                    "children": [
                        {
                            "description": "format 'Lucene50_0' ...",
                            "size_in_bytes": 2608,
                            "children" :[ ... ]
                        },
                        ...
                    ]
                },
                ...
                ]

        }
    ...
}

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