Hi team,

We are using a 10 shards (Primary / Secondary / Arbiter) cluster to host around 20k databases.
We noticed that creating new databases take lot of time, around 10s :

2017-09-11T09:48:52.763+0000 I SHARDING [conn641] distributed lock 'anthony110920175' acquired for 'createDatabase', ts : 59b65c04bb2f983e7e106b2c
2017-09-11T09:49:02.438+0000 I SHARDING [conn641] Placing [anthony110920175] on: clust-1-sh1
2017-09-11T09:49:02.448+0000 I SHARDING [conn641] distributed lock with ts: 59b65c04bb2f983e7e106b2c' unlocked.

It looks like a

db.runCommand({listDatabases:1})

is launched on all shard, one by one, before creating the database on the config servers. This command take arount 1s per shard.
Adding the "nameOnly:true" option make the query almost instantaneous. Maybe it should be added when a mongos check database existency ?

Regards,
Anthony