Azure CosmosDB number of physical partition

Question

how CosmosDB determine the number of phyiscal parition will need to store a container ?

Answer 1

Hi @sakuraime Thank you for posting your question on Microsoft Q&A. The number of physical partitions in your container depends on the following:

The number of throughput provisioned (each individual physical partition can provide a throughput of up to 10,000 request units per second). The 10,000 RU/s limit for physical partitions implies that logical partitions also have a 10,000 RU/s limit, as each logical partition is only mapped to one physical partition.

The total data storage (each individual physical partition can store up to 50GB data).
The articles below explain this in more details .
https://learn.microsoft.com/en-us/azure/cosmos-db/partitioning-overview#physical-partitions
https://learn.microsoft.com/en-us/azure/cosmos-db/scaling-provisioned-throughput-best-practices#prerequisites

Hope that helps

Regards,
Oury

Answer 2

@sakuraime you can take a look at this new doc below, which explains how many physical partitions you'll start with based on the RU/s that you set.
https://learn.microsoft.com/en-us/azure/cosmos-db/scaling-provisioned-throughput-best-practices#step-3-calculate-the-number-of-rus-to-start-with

Calculate the number of physical partitions you'll need
Number of physical partitions = Total data size in GB / Target data per physical partition in GB

Each physical partition can hold a maximum of 50 GB of storage (30 GB for Cassandra API). The value you should choose for the Target data per physical partition in GB depends on how fully packed you want the physical partitions to be and how much you expect storage to grow post-migration.

For example, if you anticipate that storage will continue to grow, you may choose to set the value to 30 GB. Assuming you've chosen a good partition key that evenly distributes storage, each partition will be ~60% full (30 GB out of 50 GB). As future data is written, it can be stored on the existing set of physical partitions, without requiring the service to immediately add more physical partitions.

In contrast, if you believe that storage will not grow significantly post-migration, you may choose to set the value higher, for example 45 GB. This means each partition will be ~90% full (45 GB out of 50 GB). This minimizes the number of physical partitions your data is spread across, which means each physical partition can get a larger fraction of the total provisioned RU/s.

Hope that help

Regards,
Oury

Answer 3

Hi @sakuraime Sorry for the delay in my response.
I initially I create a container with RU 50000, will it split to 5 physical partition already ? (even there are no data)
If you initially created with 50k RUs, you would have 7 physical partitions. (50k/6k) and then rounded up. If you scaled up to 50k after creation, then yes correct 5 physical partitions.
If I filled up some data , like 10GB , and then some day I would like to save cost and shrink the RU back to 30000
Cosmos DB doesn’t currently supporting “merging” partitions back together. Splits aren’t reversible.

Hope that helps

Regards,
Oury