Cosmos DB Partitioning and Request Units

Cosmos DB achieves elastic scale through automatic partitioning and measures all operations in Request Units (RUs). Understanding how partitioning works and how RU consumption is calculated is essential for building performant, cost-effective applications.

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How Partitioning Works

Logical Partitions

A logical partition is a set of items that share the same partition key value. For example, in an e-commerce container with partition key /customerId:

Logical Partition "c001": [Order-1, Order-2, Order-3]
Logical Partition "c002": [Order-4, Order-5]
Logical Partition "c003": [Order-6, Order-7, Order-8, Order-9]

Constraint	Limit
Max logical partition size	20 GB
Max item size	2 MB (NoSQL API)

Physical Partitions

Cosmos DB automatically maps logical partitions to physical partitions — the actual compute and storage resources:

Physical Partition 1: [c001, c002]  — 30 GB, 3000 RU/s
Physical Partition 2: [c003, c004]  — 25 GB, 3000 RU/s
Physical Partition 3: [c005, c006]  — 20 GB, 3000 RU/s

Constraint	Limit
Max physical partition size	50 GB
Max physical partition throughput	10,000 RU/s