参考：http://docs.mongodb.org/manual/reference/program/mongostat/
Fields
mongostat outputs the following fields:
inserts
The number of objects inserted into the database per second. If followed by an asterisk (e.g. *), the datum refers to a replicated operation.
query
The number of query operations per second.
update
The number of update operations per second.
delete
The number of delete operations per second.
getmore
The number of get more (i.e. cursor batch) operations per second.
command
The number of commands per second. On slave and secondary systems, mongostat presents two values separated by a pipe character (e.g. |), in the form of local|replicated commands.
flushes
The number of fsync operations per second.
mapped
The total amount of data mapped in megabytes. This is the total data size at the time of the last mongostat call.
size
The amount of virtual memory in megabytes used by the process at the time of the last mongostat call.
non-mapped
The total amount of virtual memory excluding all mapped memory at the time of the last mongostat call.
res
The amount of resident memory in megabytes used by the process at the time of the last mongostat call.
faults
Changed in version 2.1.
The number of page faults per second.
Before version 2.1 this value was only provided for MongoDB instances running on Linux hosts.
locked
The percent of time in a global write lock.
Changed in version 2.2: The locked db field replaces the locked % field to more appropriate data regarding the database specific locks in version 2.2.
locked db
New in version 2.2.
The percent of time in the per-database context-specific lock. mongostat will report the database that has spent the most time since the last mongostat call with a write lock.
This value represents the amount of time that the listed database spent in a locked state combined with the time that the mongod spent in the global lock. Because of this, and the sampling method, you may see some values greater than 100%.
idx miss
The percent of index access attempts that required a page fault to load a btree node. This is a sampled value.
qr
The length of the queue of clients waiting to read data from the MongoDB instance.
qw
The length of the queue of clients waiting to write data from the MongoDB instance.
ar
The number of active clients performing read operations.
aw
The number of active clients performing write operations.
netIn
The amount of network traffic, in bytes, received by the MongoDB instance.
This includes traffic from mongostat itself.
netOut
The amount of network traffic, in bytes, sent by the MongoDB instance.
This includes traffic from mongostat itself.
conn
The total number of open connections.
set
The name, if applicable, of the replica set.
repl
The replication status of the member.
PRI:PRIMARY
SEC:SECONDARY
REC:recovering
UNK:unknown
M: master
RTR:mongos process (“router”)
Usage
In the first example, mongostat will return data every second for 20 seconds.
mongostat -n 20 1
In many cases, using the --discover will help provide a more complete snapshot of the state of an entire group of machines. If a mongos process connected to a sharded cluster is running on port 27017 of the local machine, you can use the following form to return statistics from all members of the cluster:
mongostat --discover