mongo之db.createCollection()

db.createCollection(<name>, { capped: <boolean>,  ---固定合集，联合size,max一起考虑
                              autoIndexId: <boolean>,               ---是否自动为id字段创建索引
                              size: <number>,                             ---bytes
                              max: <number>}）                    ---rows   超限则循环覆盖

db.createCollection("log", { capped : true, size : 5242880, max : 5000 } )