MongoDB is document-based database, data is stored in JSON and XML documents. MongoDB has a document size limit of 16 MB. If the size of a JSON document exceeds 16 MB, instead of storing data as a single file, MongoDB divides the file into chunks and each chunk is stored as a document in the system. MongoDB creates a chunk of 255 KB to divide files and only the last chuck can have less than 255 KB.
MongoDB uses two collections to work with gridfs. One collection is used to store the chunk data and another collection is used to store the metadata. When you query MongoDB for the operation of the gridfs file, MongoDB uses the metadata collection to perform the query and collect data from different chunks. GridFS stores data in two collections:
chunks: Stores binary chunks.files:: Stores the file's metadata.