Reshard a Collection重分片集合

Requirements要求

Before you reshard your collection, ensure that you meet the following requirements:在重新存储集合之前，请确保满足以下要求：

• Your application can tolerate a period of two seconds where the collection that is being resharded blocks writes. 应用程序可以容忍两秒钟的时间来阻止正在重新硬存储的集合写入。During the time period where writes are blocked your application experiences an increase in latency. 在写操作被阻止的时间段内，应用程序的延迟会增加。If your workload cannot tolerate this requirement, consider refining your shard key instead.如果您的工作负载无法满足此要求，请考虑改进您的共享密钥。

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

  • Available storage space: Ensure that your available storage space is at least 1.2x the size of the collection that you want to reshard. 可用存储空间：确保可用存储空间至少是要重新硬存储的集合大小的1.2倍。For example, if the size of the collection you want to reshard is 1 TB, you should have at least 1.2 TB of free storage when starting the sharding operation.例如，如果要重新硬存储的集合大小为1 TB，则在启动分片操作时，应至少有1.2 TB的可用存储空间。
  • I/O: Ensure that your I/O capacity is below 50%.输入/输出：确保您的输入/输出容量低于50%。
  • CPU load: Ensure your CPU load is below 80%.CPU负载：确保您的CPU负载低于80%。
  Important重要

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

  • the database running out of space and shutting down数据库空间不足，正在关闭
  • decreased performance性能下降
  • the resharding operation taking longer than expected重新分片操作的时间比预期的要长

  If your application has time periods with less traffic, reshard your collection during that time if possible.如果您的应用程序具有流量较少的时间段，请尽可能在该时间段内重新硬存储您的集合。

• You have rewritten your application code to update your queries to use both the current shard key and the new shard key.您已经重写了应用程序代码，以更新查询，使其同时使用当前的分片键和新的分片键。

  The following queries return an error if the query filter does not include both the current shard key or a unique field (like _id):如果查询筛选器不同时包含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，我们建议您也重写其他查询以包含新的shard键。

  Once the resharding operation completes, you can remove the old shard key from the queries.一旦重新加密操作完成，就可以从查询中删除旧的shard密钥。

• No index builds are in progress. 没有正在进行的索引生成。Use db.currentOp() to check for any running index builds:使用db.currentOp()检查是否有任何正在运行的索引生成：

   db.adminCommand(
      {
        currentOp: true,
        $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字段值为空数组，则没有正在进行的索引生成：

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

Limitations局限性

• Only one collection can be resharded at a time.一次只能重新存储一个集合。
• writeConcernMajorityJournalDefault must be true.必须是真的。
• Resharding a collection that has a uniqueness constraint is not supported.不支持对具有唯一性约束的集合进行重新硬划分。
• The new shard key cannot have a uniqueness constraint.新的分片键不能有唯一性约束。

• The following commands and corresponding shell methods are not supported on the collection that is being resharded while the resharding operation is in progress:正在进行重新存储操作时，正在重新存储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:正在进行重新存储操作时，群集上不支持以下命令和方法：

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

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

• If the collection to be resharded uses Atlas Search, the search index will become unavailable when the resharding operation completes. 如果要重新存储的集合使用Atlas Search，则当重新存储操作完成时，搜索索引将不可用。You need to manually rebuild the search index once the resharding operation completes.重新存储操作完成后，需要手动重建搜索索引。

Resharding Process重分片过程

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

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>,
    ...
  },
  ...
]

{
  ok: 1,
  '$clusterTime': {
    clusterTime: <timestamp>,
    signature: {
      hash: Binary(Buffer.from("0000000000000000000000000000000000000000", "hex"), 0),
      keyId: <number>
    }
  },
  operationTime: <timestamp>
}

sh.status()

databases
[
  {
     database: {
     _id: '<database>',
     primary: '<shard>',
     partitioned: true,
     version: {
       uuid: <uuid>,
       timestamp: <timestamp>,
       lastMod: <number>
     }
   },
   collections: {
     '<database>.<collection>': {
       shardKey: <shardkey>,
       unique: <boolean>,
       balancing: <boolean>,
       chunks: [],
       tags: []
     }
   }
  }
  ...
]

db.adminCommand({
  commitReshardCollection: "<database>.<collection>"
})

db.adminCommand({
  abortReshardCollection: "<database>.<collection>"
})

Behavior行为

Error Cases错误案例

db.runCommand({
  cleanupReshardCollection: "<database>.<collection>"
})