convertToCapped

Command Fields

The command takes the following fields:

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.

Convert a Collection

The following example uses db.collection.insertOne() to create an events collection, and db.collection.stats() to obtain information about the collection:

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

MongoDB will return the following:

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

To convert the events collection into a capped collection and view the updated collection information, run the following commands:

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

MongoDB will return the following:

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

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.