Troubleshoot Shard Keys
The ideal shard key allows MongoDB to distribute documents evenly throughout the cluster while facilitating common query patterns. A suboptimal shard key can lead to the following problems:
In the following you can find out more about common problems with shard keys and how to resolve them.
Jumbo Chunks
If you are seeing jumbo chunks, either the cardinality of your shard key is insufficient or the frequency of the shard key values is unevenly distributed.
To increase the cardinality of your shard key or change the distribution of your shard key values, you can:
refine your shard key by adding a suffix field or fields to the existing key to increase cardinality
reshard your collection using a different shard key with higher cardinality
To only change the distribution of your shard key values, you can also consider using Hashed Sharding to distribute your data more evenly.
For advice on choosing a shard key see Choose a Shard Key.
Uneven Load Distribution
If your cluster is experiencing uneven load distribution, check if your shard key increases monotonically. A shard key that is a monotonically increasing field, leads to an uneven read and write distribution.
Consider an orders collection that is sharded on an order_id
field. The order_id is an integer which increases by one with each
order.
New documents are generally written to the same shard and chunk. The shard and chunk that receive the writes are called hot shard and hot chunk. The hot shard changes over time. When chunks are split, the hot chunk moves to a different shard to optimize data distribution.
If users are more likely to interact with recent orders, which are all on the same shard, the shard that contains recent orders will receive most of the traffic.
If you have a monotonically increasing shard key, consider resharding your collection. For advice on choosing a shard key see Choose a Shard Key.
If your data model requires sharding on a key that changes monotonically, consider using Hashed Sharding.
Decreased Query Performance Over Time
If you are noticing decreased query performance over time, it is possible that your cluster is performing scatter-gather queries.
To evaluate if your cluster is performing scatter-gather queries, check if your most common queries include the shard key.
If you include the shard key in your queries, check if your shard key is hashed. With Hashed Sharding, documents are not stored in ascending or descending order of the shard key field value. Performing range based queries on the shard key value on data that is not stored in ascending or descending order results in less performant scatter-gather queries. If range based queries on your shard key are a common access pattern, consider resharding your collection.
If you do not include the shard key in your most common queries, it is possible that you could increase performance by resharding your collection. For advice on choosing a shard key see Choose a Shard Key.