用最简单的术语来说，碎片只是存储在磁盘上一个分离文件夹中的索引的一部分:

这个截图显示了整个Elasticsearch目录。

如您所见，所有数据都进入data目录。

通过检查索引C-mAfLltQzuas72iMiIXNw，我们看到它有五个碎片(文件夹0到4)。

另一方面，JH_A8PgCRj-GK0GeQ0limw索引只有一个碎片(0文件夹)。

pri表示碎片的总数。

碎片:

Being distributed search server, ElasticSearch uses concept called Shard to distribute index documents across all nodes. An index can potentially store a large amount of data that can exceed the hardware limits of a single node For example, a single index of a billion documents taking up 1TB of disk space may not fit on the disk of a single node or may be too slow to serve search requests from a single node alone. To solve this problem, Elasticsearch provides the ability to subdivide your index into multiple pieces called shards. When you create an index, you can simply define the number of shards that you want. Documents are stored in shards, and shards are allocated to nodes in your cluster As your cluster grows or shrinks, Elasticsearch will automatically migrate shards between nodes so that the cluster remains balanced. A shard can be either a primary shard or a replica shard. Each document in your index belongs to a single primary shard, so the number of primary shards that you have determines the maximum amount of data that your index can hold A replica shard is just a copy of a primary shard.

副本:

Replica shard is the copy of primary Shard, to prevent data loss in case of hardware failure. Elasticsearch allows you to make one or more copies of your index’s shards into what are called replica shards, or replicas for short. An index can also be replicated zero (meaning no replicas) or more times. The number of shards and replicas can be defined per index at the time the index is created. After the index is created, you may change the number of replicas dynamically anytime but you cannot change the number of shards after-the-fact. By default, each index in Elasticsearch is allocated 5 primary Shards and 1 replica which means that if you have at least two nodes in your cluster, your index will have 5 primary shards and another 5 replica shards (1 complete replica) for a total of 10 shards per index.

索引被分解成碎片，以便分布它们和扩展它们。

副本是碎片的副本。

节点是弹性搜索的一个运行实例，属于一个集群。

集群由一个或多个具有相同集群名称的节点组成。每个集群都有一个由集群自动选择的主节点，如果当前的主节点发生故障，可以将其替换。

如果你真的不喜欢看到它变黄。您可以将副本的数量设置为0:

curl -XPUT 'localhost:9200/_settings' -d '
{
    "index" : {
        "number_of_replicas" : 0
    }
}
'

请注意，您应该只在本地开发框上执行此操作。

I will explain this using a real word scenarios. Imagine you are a running a ecommerce website. As you become more popular more sellers and products add to your website. You will realize the number of products you might need to index has grown and it is too large to fit in one hard disk of one node. Even if it fits in to hard disk, performing a linear search through all the documents in one machine is extremely slow. one index on one node will not take advantage of the distributed cluster configuration on which the elasticsearch works.

So elasticsearch splits the documents in the index across multiple nodes in the cluster. Each and every split of the document is called a shard. Each node carrying a shard of a document will have only a subset of the document. suppose you have 100 products and 5 shards, each shard will have 20 products. This sharding of data is what makes low latency search possible in elasticsearch. search is conducted parallel on multiple nodes. Results are aggregated and returned. However the shards doesnot provide fault tolerance. Meaning if any node containing the shard is down, the cluster health becomes yellow. Meaning some of the data is not available.

To increase the fault tolerance replicas come in to picture. By deault elastic search creates a single replica of each shard. These replicas are always created on a other node where the primary shard is not residing. So to make the system fault tolerant, you might have to increase the number of nodes in your cluster and it also depends on number of shards of your index. The general formula to calculate the number of nodes required based on replicas and shards is "number of nodes = number of shards*(number of replicas + 1)".The standard practice is to have atleast one replica for fault tolerance.

设置碎片数量是一个静态操作，这意味着您必须在创建索引时指定它。在此之后的任何改变都需要完全重新索引数据，并且需要时间。但是，副本数量的设置是一个动态操作，也可以在索引创建后的任何时间完成。

您可以使用下面的命令为索引设置碎片和副本的数量。

curl -XPUT 'localhost:9200/sampleindex?pretty' -H 'Content-Type: application/json' -d '
{
  "settings":{
    "number_of_shards":2,
    "number_of_replicas":1
  }
}'

在ElasticSearch中，在顶层，我们将文档索引为索引。每个索引都有若干个分片，这些分片内部分布数据，而这些分片内部存在Lucene段，这是数据的核心存储。因此，如果索引有5个分片，这意味着数据已经分布在各个分片上，并且分片中存在不同的数据。

请观看解释ES核心的视频 https://www.youtube.com/watch?v=PpX7J-G2PEo

关于多索引或多碎片的文章 弹性搜索，多个索引vs不同数据集的一个索引和类型?