MongoDB indexing - mongodb

Question

We have a MongoDB collection containing nearly 40 million records. The current size of the collection is 5GB. The data stored in this collection contains following fields:
_id: "MongoDB id"
userid: "user id" (int)
mobile: "users mobile number" (int)
transaction: "transaction id" (int)
sms: "message sent to user mobile" (text)
created_dt: "unix timestamp of the transaction"
Apart from the index on _id (created by default), we have defined separate indexes on the mobile and transaction fields.
However, the following query takes anywhere between 60 to 120 seconds to complete:
{
mobile:<users mobile number>
}
I access MongoDB using RockMongo. MongoDB is hosted on a server with 16GB RAM. Nearly 8GB RAM on this server is free.
What is it that I am doing wrong here?
Update:
Output of explain:
{
"cursor" : "BasicCursor",
"nscanned" : 37145516,
"nscannedObjects" : 37145516,
"n" : 37145516,
"millis" : 296040,
"nYields" : 1343,
"nChunkSkips" : 0,
"isMultiKey" : false,
"indexOnly" : false,
"indexBounds" : {
}
}
Output of mongostat at the time of the query
insert query update delete getmore command flushes mapped vsize res faults locked % idx miss % qr|qw ar|aw netIn netOut conn time
13 2 0 0 0 1 0 168g 336g 6.86g 1 1 0 0|0 1|0 21k 1k 19 11:30:04
16 0 0 0 0 1 0 168g 336g 6.88g 0 0.1 0 0|0 1|0 21k 1k 19 11:30:05
14 0 0 0 0 1 0 168g 336g 6.86g 0 0 0 0|0 1|0 29k 1k 19 11:30:06
10 0 0 0 0 1 0 168g 336g 6.86g 0 0 0 0|0 1|0 19k 1k 19 11:30:07
16 0 0 0 0 1 0 168g 336g 6.88g 0 0.1 0 0|0 1|0 21k 1k 19 11:30:08
9 0 0 0 0 1 0 168g 336g 6.89g 0 0 0 0|0 1|0 13k 1k 19 11:30:09
19 0 0 0 0 1 0 168g 336g 6.89g 0 0 0 0|0 1|0 27k 1k 19 11:30:10
12 0 0 0 0 1 0 168g 336g 6.89g 1 1.2 0 0|0 1|0 24k 1k 19 11:30:11
17 0 0 0 0 1 0 168g 336g 6.89g 1 1.7 0 0|0 1|0 31k 1k 19 11:30:12
15 0 0 0 0 1 0 168g 336g 6.89g 0 0 0 0|0 1|0 19k 1k 19 11:30:13
Update 2:
Until recently, we used to store another collection with about 1.3 billion documents in the same MongoDB server. This collection has now been removed (dropped). This may explain the mapped / vsize column in above output from mongostat.
The server also stores 6 other collections which have frequent inserts. The total storage size currently is about 35GB.
Update 3:
Indexes defined on the collection. Created using RockMongo.
[
{
"v" : 1,
"key" : {
"_id" : 1
},
"ns" : "mymongodb.transaction_sms_details",
"name" : "_id_"
},
{
"v" : 1,
"key" : {
"_transaction_mobile_" : 1
},
"ns" : "mymongodb.transaction_sms_details",
"background" : 1,
"name" : "mobile"
},
{
"v" : 1,
"key" : {
"_transaction_transaction_" : 1
},
"ns" : "mymongodb.transaction_sms_details",
"background" : 1,
"name" : "transaction"
}
]

Answer 1

The keys generated by RockMongo is apparently incorrect.
"_transaction_mobile_" : 1
"_transaction_transtion_" : 1
I don't know what's wrong with RockMongo, but I think this can fix the issue:
db.xxx.dropIndexes();
db.xxx.ensureIndex({mobile: 1});
db.xxx.ensureIndex({transaction: 1});
Notice: This may take VERY LONG time. Don't do this on a running production machine.