Reshard a Collection重新分片集合

About this Task关于此任务

• Only one collection can be resharded at a time.一次只能重新分片一个集合。
• writeConcernMajorityJournalDefault must be true.必须true。
• To reshard a collection that has a uniqueness constraint, the new shard key must satisfy the unique index requirements for any existing unique indexes.要重新分片具有唯一性约束的集合，新的分片键必须满足任何现有唯一索引的唯一索引要求。

• The following commands and corresponding shell methods are not supported on the collection that is being resharded while the resharding operation is in progress:在重新分片操作进行期间，正在重新分片的集合不支持以下命令和相应的shell方法：

  • collMod
  • convertToCapped
  • createIndexes
  • createIndex()
  • drop
  • drop()
  • dropIndexes
  • dropIndex()
  • renameCollection
  • renameCollection()

• The following commands and methods are not supported on the cluster while the resharding operation is in progress:正在进行重新分片操作时，群集不支持以下命令和方法：

  • addShard
  • removeShard
  • db.createCollection()
  • dropDatabase

  Warning

  Using any of the preceding commands during a resharding operation causes the resharding operation to fail.在重新分片操作期间使用上述任何命令都会导致重新分片操作失败。

• If the collection you're resharding uses MongoDB Search, the search index becomes unavailable when the resharding operation completes. 如果您正在重新分片的集合使用MongoDB搜索，则重新分片操作完成后，搜索索引将不可用。You need to manually rebuild the search index once the resharding operation completes.重新分片操作完成后，您需要手动重建搜索索引。

Before you Begin开始之前

Before you reshard your collection, ensure that you meet the following requirements:在重新分片集合之前，请确保您符合以下要求：

• Your application can tolerate a period of two seconds where the affected collection blocks writes. During the time period where writes are blocked, your application experiences an increase in latency.应用程序可以容忍受影响的集合块写入的两秒钟时间。在写入被阻止的时间段内，应用程序的延迟会增加。

  If your workload cannot tolerate this requirement, consider refining your shard key instead.如果工作负载不能容忍这一要求，可以考虑改进分片键。

• Your database meets these resource requirements:数据库满足以下资源要求：

  • Ensure that the available storage space on each shard the collection will be distributed across is at least twice the size of the collection that you want to reshard and its total index size, divided by the number of shards.确保集合将分布在每个分片上的可用存储空间至少是要重新分片的集合大小及其总索引大小除以分片数量的两倍。

    storage_req = ( ( collection_storage_size + index_size ) * 2 ) / shard_count

    For example, consider a collection that contains 2 TB of data and has a 400 GB index distributed across four shards. To perform a resharding operation on this collection, each shard would require 1.2 TB of available storage.例如，考虑一个包含2TB数据的集合，其400GB索引分布在四个分片上。要对此集合执行重新分片操作，每个分片需要1.2 TB的可用存储空间。

    1.2 TB storage = ( ( 2 TB collection + 0.4 TB index ) * 2 ) / 4 shards

    To meet storage requirements, you may need to upgrade to the next tier of storage during the resharding operation. You can scale down once the operation completes.为了满足存储要求，您可能需要在重新分片操作期间升级到下一层存储。操作完成后，您可以缩小规模。

  • Ensure that your I/O capacity is below 50%.确保I/O容量低于50%。
  • Ensure that your CPU load is below 80%.确保CPU负载低于80%。

  Important

  These requirements are not enforced by the database. A failure to allocate enough resources can result in:数据库不强制执行这些要求。未能分配足够的资源可能会导致：

  • the database running out of space and shutting down数据库空间不足并关闭
  • decreased performance性能下降
  • the operation taking longer than expected手术时间比预期的要长

  If your application has time periods with less traffic, perform this operation on the collection during that time if possible.如果应用程序有流量较少的时间段，请在可能的情况下在该时间段内对集合执行此操作。

• You must rewrite your application's queries to use both the current shard key and the new shard key.您必须重写应用程序的查询，以使用当前分片键和新分片键。

  Tip

  If your application can tolerate downtime, you can perform these steps to avoid rewriting your application's queries to use both the current and new shard keys:如果应用程序可以容忍停机时间，你可以执行以下步骤来避免重写应用程序的查询，以使用当前和新的分片键：

  1. Stop your application.停止应用程序。
  2. Rewrite your application to use the new shard key.重写应用程序以使用新的分片键。
  3. Wait until resharding completes. To monitor the resharding process, use the $currentOp pipeline stage.等待重新分片完成。要监视重新分片过程，请使用$currentOp管道阶段。
  4. Deploy your rewritten application.部署重写的应用程序。

  Before resharding completes, the following queries return an error if the query filter does not include either the current shard key or a unique field (like _id):在重新分片完成之前，如果查询筛选器不包括当前分片键或唯一字段（如_id），则以下查询将返回错误：

  • deleteOne()
  • findAndModify()
  • findOneAndDelete()
  • findOneAndReplace()
  • findOneAndUpdate()
  • replaceOne()
  • updateOne()

  For optimal performance, we recommend that you also rewrite other queries to include the new shard key.为了获得最佳性能，我们建议您还重写其他查询以包含新的分片键。

  Once the resharding operation completes, you can remove the old shard key from the queries.重新分片操作完成后，您可以从查询中删除旧的分片键。

• No index builds are in progress. To check for running index builds, use $currentOp:没有正在进行的索引构建。要检查是否正在运行索引构建，请使用$currentOp：

  db.getSiblingDB("admin").aggregate( [
     { $currentOp : { idleConnections: true } },
     { $match: {
           $or: [
               { "op": "command", "command.createIndexes": { $exists: true } },
               { "op": "none", "msg": /^Index Build/ }
           ]
        }
     }
  ] )

  In the result document, if the inprog field value is an empty array, there are no index builds in progress:在结果文档中，如果inprog字段值是一个空数组，则没有正在进行的索引构建：

  {
     inprog: [],
     ok: 1,
     '$clusterTime': { ... },
     operationTime: <timestamp>
  }

Steps步骤

In a collection resharding operation, a shard can be a:在集合重新分片操作中，分片可以是：

• donor, which currently stores chunks for the sharded collection.捐赠者，目前为分片集合存储块。
• recipient, which stores new chunks for the sharded collection based on the shard keys and zones.接收者，它根据分片键和区域为分片集合存储新块。

A shard can be donor and a recipient at the same time.一个分片可以同时作为捐赠者和接受者。

The config server primary is always the resharding coordinator and starts each phase of the resharding operation.配置服务器主服务器始终是重新分片协调器，并启动重新分片操作的每个阶段。

db.adminCommand({
  reshardCollection: "<database>.<collection>",
  key: <shardkey>
})

db.adminCommand({
  reshardCollection: "<database>.<collection>",
  key: <shardkey>,
  forceRedistribution: true
})

db.getSiblingDB("admin").aggregate([
  { $currentOp: { allUsers: true, localOps: false } },
  {
    $match: {
      type: "op",
      "originatingCommand.reshardCollection": "<database>.<collection>"
    }
  }
])

[
  {
    shard: '<shard>',
    type: 'op',
    desc: 'ReshardingRecipientService | ReshardingDonorService | ReshardingCoordinatorService <reshardingUUID>',
    op: 'command',
    ns: '<database>.<collection>',
    originatingCommand: {
      reshardCollection: '<database>.<collection>',
      key: <shardkey>,
      unique: <boolean>,
      collation: { locale: 'simple' }
    },
    totalOperationTimeElapsedSecs: <number>,
    remainingOperationTimeEstimatedSecs: <number>,
    ...
  },
  ...
]

Behavior行为

Error Case错误案例