convertToCapped¶

convertToCapped¶

Do Not Run This Command In Sharded Clusters

MongoDB does not support the convertToCapped command in a sharded cluster.

The convertToCapped command converts an existing, non-capped collection to a capped collection within the same database.

The command has the following syntax:语法如下所示：

{ convertToCapped: <collection>,
  size: <capped size>,
  writeConcern: <document>,
  comment: <any>
}

The command takes the following fields:

Field字段	Description描述
convertToCapped	The name of the existing collection to convert.
size	The maximum size, in bytes, for the capped collection.
writeConcern	Optional.可选。A document expressing the write concern of the drop command. Omit to use the default write concern.
comment	Optional.可选。A user-provided comment to attach to this command. Once set, this comment appears alongside records of this command in the following locations: mongod log messages, in the attr.command.cursor.comment field. Database profiler output, in the command.comment field. currentOp output, in the command.comment field. A comment can be any valid BSON type (string, integer, object, array, etc). New in version 4.4.版本4.4中的新功能。

convertToCapped takes an existing collection (<collection>) and transforms it into a capped collection with a maximum size in bytes, specified by the size argument (<capped size>).

During the conversion process, the convertToCapped command exhibits the following behavior:

• MongoDB traverses the documents in the original collection in natural order and loads the documents into a new capped collection.
• If the capped size specified for the capped collection is smaller than the size of the original uncapped collection, then MongoDB will overwrite documents in the capped collection based on insertion order, or first in, first out order.
• Internally, to convert the collection, MongoDB uses the following procedure
  • cloneCollectionAsCapped command creates the capped collection and imports the data.
  • MongoDB drops the original collection.
  • renameCollection renames the new capped collection to the name of the original collection.
• This holds a database exclusive lock for the duration of the operation. Other operations which lock the same database will be blocked until the operation completes. See What locks are taken by some common client operations? for operations that lock the database.

Warning

The convertToCapped will not recreate indexes from the original collection on the new collection, other than the index on the _id field. If you need indexes on this collection you will need to create these indexes after the conversion is complete.

db.events.save( { click: 'button-1', time: new Date() } )
db.events.stats()

{
        "ns" : "test.events",
        ...
        "capped" : false,
        ...
}

db.runCommand( { convertToCapped: 'events', size: 8192 } )
db.events.stats()

{
     "ns" : "test.events",
     ...
     "capped" : true,
     "max" : NumberLong("9223372036854775807"),
     "maxSize" : 8192,
     ...
}