I wanted to backup my database, and then execute the following command:

mongodump -d sandbox --out .

But very, very slow。

I saw the mongodb log, such a large number of logs:
Tue Sep 10 23:00:14 [conn2409] getmore sandbox.followbrand_flwrs query: { query: {}, $snapshot: true } cursorid:7315171840358601840 ntoreturn:0 exhaust:1 keyUpdates:0 numYields: 3645 locks(micros) r:10464765 nreturned:6993 reslen:4194538 36678ms
Tue Sep 10 23:00:50 [conn2409] getmore sandbox.followbrand_flwrs query: { query: {}, $snapshot: true } cursorid:7315171840358601840 ntoreturn:0 exhaust:1 keyUpdates:0 numYields: 3602 locks(micros) r:10267399 nreturned:7015 reslen:4194822 35893ms

The indexes is:

> db.followbrand_flwrs.getIndexes()
[
{
"v" : 1,
"key" :

,
"ns" : "sandbox_qqonline.followbrand_flwrs",
"name" : "id"
},
{
"v" : 1,
"key" :

,
"ns" : "sandbox_qqonline.followbrand_flwrs",
"name" : "update_time_1"
}
]

> db.followbrand_flwrs.count()
21579466