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 } )