Sharded MongoDB monitoring: the metrics that predict an imbalance

Sharded MongoDB is a different beast from a replica set. The dashboards you have show per-shard health; the failures usually come from between the shards. Imbalance, jumbo chunks, balancer livelock, and shard-key skew don’t show up on a per-shard graph. Here’s the cluster-level monitoring set you actually need.

Cluster-level metrics

// On any mongos:
db.adminCommand({ balancerStatus: 1 })
sh.status()
db.getSiblingDB("config").chunks.aggregate([
  { $group: { _id: "$shard", chunks: { $sum: 1 } } }
])

That last one is the one that surprises teams: the chunk count by shard. Healthy clusters are within ±5%; problematic ones are 30%+ apart and the balancer has given up.

Balancer health

• Round time — balancer rounds typically take seconds. Rounds taking minutes mean a single migration is stuck.
• Migrations failed (last hour) — should be 0; sustained failures = chunk that won’t move (jumbo, write conflict, lock contention).
• Active migrations — only 1 per source shard at a time; if 0 for 24h on an imbalanced cluster, balancer is paused or stuck.

Jumbo chunks

A chunk that exceeds chunkSize (default 128MB) and can’t be split is marked jumbo. The balancer refuses to migrate it. Find them:

db.getSiblingDB("config").chunks.find({ jumbo: true })
  .sort({ "min": 1 })
  .toArray()

The fix depends on cause: bad shard key (the most common — can’t fix without resharding), or a single document > chunk size (rare; consider splitting the document). On 4.4+, refineCollectionShardKey can help by adding suffix fields to make ranges splittable.

Shard-key skew

Hot shard means your shard key concentrates writes onto one range. Test:

// Top 10 shard keys by write rate over the last hour
db.serverStatus().opcounters
// per-shard, computed via mongotop or collected via metrics scrape

// Compare ops/sec across shards. > 2× spread = hot shard.

Recovery options, in increasing intrusiveness: hash existing key, refine the shard key, reshard (online resharding is GA from 5.0). All require planning.

Alerts that matter

• Chunk count spread > 30% across shards
• Balancer failed migrations > 0 sustained 1h
• Any chunk marked jumbo (immediate)
• Per-shard ops/sec spread > 2× median
• Per-shard storage spread > 2× median
• config.chunks rebalance round time > 5min sustained

FAQ