It counts on fs.files and fs.chunks every time to build index on collections when new a GridFS object.

Source code in com.mongodb.gridfs.GirdFS :

// ensure standard indexes as long as collections are small
try {
if (_filesCollection.count() < 1000)

{ _filesCollection.ensureIndex( BasicDBObjectBuilder.start().add( "filename" , 1 ).add( "uploadDate" , 1 ).get() ); }

if (_chunkCollection.count() < 1000)

{ _chunkCollection.ensureIndex( BasicDBObjectBuilder.start().add( "files_id" , 1 ).add( "n" , 1 ).get() , BasicDBObjectBuilder.start().add( "unique" , true ).get() ); }

} catch (MongoException e)

{ LOGGER.info(String.format("Unable to ensure indices on GridFS collections in database %s", db.getName())); }

I think it should check on the index exists ,not the row counts .