mongodb on WSLが起動しなかった

http://github.com/nesosuke/bookmanagerをつくっていて，mongoDBが立ち上がらなくてクネクネしたのでメモ．

$ mongo
MongoDB shell version v3.6.3
connecting to: mongodb://127.0.0.1:27017
2021-07-18T21:17:57.153+0900 W NETWORK  [thread1] Failed to connect to 127.0.0.1:27017, in(checking socket for error after poll), reason: Connection refused
2021-07-18T21:17:57.153+0900 E QUERY    [thread1] Error: couldn't connect to server 127.0.0.1:27017, connection attempt failed :
connect@src/mongo/shell/mongo.js:251:13
@(connect):1:6
exception: connect failed

 sudo mongod --fork --config /etc/mongodb.conf 

をしたらなぜか起動する（ソースは忘れた）ので備忘録．