GridFS Reference

GridFS stores files in two collections:

GridFS places the collections in a common bucket by prefixing each with the bucket name. By default, GridFS uses two collections with names prefixed by fs bucket:

  • fs.files
  • fs.chunks

You can choose a different bucket name than fs, and create multiple buckets in a single database.

See also

GridFS for more information about GridFS.

The chunks Collection

Each document in the chunks collection represents a distinct chunk of a file as represented in the GridFS store. The following is a prototype document from the chunks collection.:

  "_id" : <ObjectId>,
  "files_id" : <ObjectId>,
  "n" : <num>,
  "data" : <binary>

A document from the chunks collection contains the following fields:


The unique ObjectId of the chunk.


The _id of the “parent” document, as specified in the files collection.


The sequence number of the chunk. GridFS numbers all chunks, starting with 0.


The chunk’s payload as a BSON binary type.

The chunks collection uses a compound index on files_id and n, as described in GridFS Index.

The files Collection

Each document in the files collection represents a file in the GridFS store. Consider the following prototype of a document in the files collection:

  "_id" : <ObjectId>,
  "length" : <num>,
  "chunkSize" : <num>,
  "uploadDate" : <timestamp>,
  "md5" : <hash>,

  "filename" : <string>,
  "contentType" : <string>,
  "aliases" : <string array>,
  "metadata" : <dataObject>,

Documents in the files collection contain some or all of the following fields. Applications may create additional arbitrary fields:


The unique ID for this document. The _id is of the data type you chose for the original document. The default type for MongoDB documents is BSON ObjectId.


The size of the document in bytes.


The size of each chunk. GridFS divides the document into chunks of the size specified here. The default size is 255 kilobytes.

Changed in version 2.4.10: The default chunk size changed from 256k to 255k.


The date the document was first stored by GridFS. This value has the Date type.


An MD5 hash returned by the filemd5 command. This value has the String type.


Optional. A human-readable name for the document.


Optional. A valid MIME type for the document.


Optional. An array of alias strings.


Optional. Any additional information you want to store.

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