Master MongoDB Monitoring: Using mongostat and db.stats() for Performance Insights

1. mongostat Tool Overview

mongostat is a MongoDB‑provided command‑line utility that periodically reports the current state of a MongoDB instance. It is useful for quickly spotting slowdowns or other issues.

mongostat Command Options

--help : display help information

--version : show MongoDB version

--host : specify the host (and optionally the port)

--port : specify the port if not included in --host --sslCAFile : path to the CA certificate

--username : username for authentication

--password : password for authentication

Typical usage example:

mongostat --rowcount 2 --host 192.168.0.8:27027

Output Metrics Explanation

inserts/s : inserts per second

query/s : queries per second

update/s : updates per second

delete/s : deletes per second

getmore/s : getmore operations per second

command/s : total commands per second (includes all above)

flushes/s : fsync operations per second

mapped/s : amount of data memory‑mapped (MB)

vsize : virtual memory size (MB)

res : resident (physical) memory size (MB)

faults/s : page faults per second (Linux only)

locked % : percentage of time the database is locked (aim < 50%)

idx miss % : index miss percentage

qt|r|w : queue lengths for total, read, and write operations

ar : active client write operations

conn : current connections

time : timestamp

2. MongoDB Built‑in Functions

db.stats() Function

Provides database‑level statistics such as collection count, object count, average object size, data size, storage size, index information, and more.

use test
db.stats()
{
  "collections" : 9,
  "objects" : 4278845,
  "avgObjSize" : 224.56603031892953,
  "dataSize" : 960883236,
  "storageSize" : 1195438080,
  "numExtents" : 59,
  "indexes" : 13,
  "indexSize" : 801931264,
  "fileSize" : 6373244928,
  "ok" : 1
}

Key output fields:

collections : number of collections

objects : estimated total document count

avgObjSize : average document size (bytes)

dataSize : total size of all document data (bytes)

storageSize : allocated storage size on disk (bytes)

indexes : number of indexes

indexSize : total size of indexes (bytes)

fileSize : pre‑allocated file size for the database

db.serverStatus() Function

Returns a comprehensive snapshot of the server’s status, including version, uptime, memory usage, connection counts, lock statistics, index counters, background flushing metrics, cursor information, replication role, operation counters, and assertion counts.

{
  "version" : "1.6.5",
  "uptime" : 7208469,
  "mem" : {
    "bits" : 64,
    "resident" : 3131,
    "virtual" : 6172,
    "mapped" : 4927
  },
  "connections" : {
    "current" : 402,
    "available" : 2599
  },
  "globalLock" : {
    "totalTime" : 7208469556704,
    "lockTime" : 4959693717,
    "ratio" : 0.000688036992871448,
    "currentQueue" : { "total" : 0, "readers" : 0, "writers" : 0 }
  },
  "indexCounters" : { "btree" : { "accesses" : 2821726, "hits" : 2821725, "misses" : 1, "missRatio" : 3.543930204420982e-7 } },
  "backgroundFlushing" : { "flushes" : 120133, "total_ms" : 73235923, "average_ms" : 609.6236920746173, "last_ms" : 1332 },
  "opcounters" : { "insert" : 269351, "query" : 19331151, "update" : 14199331, "delete" : 1, "getmore" : 145575, "command" : 55982302 },
  "asserts" : { "regular" : 0, "warning" : 0, "msg" : 0, "user" : 27, "rollovers" : 0 },
  "ok" : 1
}

Important fields to monitor:

Host, version, process : basic instance identification

Uptime / uptimeEstimate : how long the server has been running

globalLock.* : lock time and ratio, queue lengths

mem.resident / mem.virtual / mem.mapped : memory usage details

connections.current / connections.available : active and available connections

indexCounters.btree.* : index access statistics

backgroundFlushing.* : disk flush frequency and latency

opcounters.* : cumulative operation counts since startup

asserts.* : assertion occurrences

By regularly reviewing these metrics, administrators can quickly identify performance bottlenecks, memory pressure, lock contention, and other issues affecting MongoDB stability.