cloneCollectionAsCapped

Definition定义

cloneCollectionAsCapped

The cloneCollectionAsCapped command creates a new capped collection from an existing, non-capped collection within the same database. cloneCollectionAsCapped命令从同一数据库中的现有非封顶集合创建新的封顶集合。The operation does not affect the original non-capped collection.该操作不会影响原始的无上限集合。

The command has the following syntax:该命令具有以下语法：

{ cloneCollectionAsCapped: <existing collection>,
  toCollection: <capped collection>,
  size: <capped size>,
  writeConcern: <document>,
  comment: <any>
}

The command takes the following fields:该命令接受以下字段：

Field字段	Description描述
cloneCollectionAsCapped	The name of the existing collection to copy.要复制的现有集合的名称。
toCollection	The name of the new capped collection to create.要创建的新封顶集合的名称。 The name of the new capped collection must be distinct and cannot be the same as that of the original existing collection.新封顶集合的名称必须不同，不能与原始现有集合的名称相同。
size	The maximum size,in bytes, for the capped collection.上限集合的最大大小（字节）。
writeConcern	Optional. 可选。A document expressing the write concern of the drop command. 表示drop命令的写入关注的文档。Omit to use the default write concern.忽略则使用默认的写入问题。
comment	Optional. 可选。A user-provided comment to attach to this command. 用户提供了附加到此命令的注释。Once set, this comment appears alongside records of this command in the following locations:设置后，此注释将与此命令的记录一起显示在以下位置： mongod log messages, in the attr.command.cursor.comment field.mongod日志消息，在attr.command.cursor.comment字段中。 Database profiler output, in the command.comment field.数据库探查器输出，在command.comment字段中。 currentOp output, in the command.comment field.currentOp输出，在command.comment字段中。 A comment can be any valid BSON type(string, integer, object, array, etc).注释可以是任何有效的BSON类型（字符串、整数、对象、数组等）。 New in version 4.4.在版本4.4中新增。

The command copies an existing collection and creates a new capped collection with a maximum size specified by the capped size in bytes.该命令复制现有集合并创建一个新的封顶集合，其最大大小由封顶大小（以字节为单位）指定。

To replace the original non-capped collection with a capped collection, use the convertToCapped command.要用封顶集合替换原始的非封顶集合，请使用convertToCapped命令。

Behavior行为

If the capped size is less than the size of the source collection, then not all documents in the source collection will exist in the destination capped collection.如果封顶大小小于源集合的大小，则目标上限集合中不会存在源集合中的所有文档。

This holds a database exclusive lock for the duration of the operation. 这将在操作期间保持数据库独占锁定。Other operations which lock the same database will be blocked until the operation completes. 锁定同一数据库的其他操作将被阻止，直到操作完成。See What locks are taken by some common client operations? for operations that lock the database.参阅一些常见的客户端操作使用了哪些锁？以了解锁定数据库的操作。