Managing Amazon Aurora PostgreSQL - Amazon Aurora
Scaling Aurora PostgreSQL DB instancesMaximum connectionsTemporary storage limits

Managing Amazon Aurora PostgreSQL

The following section discusses managing performance and scaling for an Amazon Aurora PostgreSQL DB cluster. It also includes information about other maintenance tasks.

Topics

Scaling Aurora PostgreSQL DB instances

You can scale Aurora PostgreSQL DB instances in two ways, instance scaling and read scaling. For more information about read scaling, see Read scaling.

You can scale your Aurora PostgreSQL DB cluster by modifying the DB instance class for each DB instance in the DB cluster. Aurora PostgreSQL supports several DB instance classes optimized for Aurora. Don't use db.t2 or db.t3 instance classes for larger Aurora clusters of size greater than 40 terabytes (TB).

Scaling isn't instantaneous. It can take 15 minutes or more to complete the change to a different DB instance class. If you use this approach to modify the DB instance class, you apply the change during the next scheduled maintenance window (rather than immediately) to avoid affecting users.

As an alternative to modifying the DB instance class directly, you can minimize downtime by using the high availability features of Amazon Aurora. First, add an Aurora Replica to your cluster. When creating the replica, choose the DB instance class size that you want to use for your cluster. When the Aurora Replica is synchronized with the cluster, you then failover to the newly added Replica. To learn more, see Aurora Replicas and Fast failover with Amazon Aurora PostgreSQL.

For detailed specifications of the DB instance classes supported by Aurora PostgreSQL, see Supported DB engines for DB instance classes.

Maximum connections to an Aurora PostgreSQL DB instance

An Aurora PostgreSQL DB cluster allocates resources based on the DB instance class and its available memory. Each connection to the DB cluster consumes incremental amounts of these resources, such as memory and CPU. Memory consumed per connection varies based on query type, count, and whether temporary tables are used. Even an idle connection consumes memory and CPU. That's because when queries run on a connection, more memory is allocated for each query and it's not released completely, even when processing stops. Thus, we recommend that you make sure your applications aren't holding on to idle connections: each one of these wastes resources and affects performance negatively. For more information, see Resources consumed by idle PostgreSQL connections.

The maximum number of connections allowed by an Aurora PostgreSQL DB instance is determined by the max_connections parameter value specified in the parameter group for that DB instance. The ideal setting for the max_connections parameter is one that supports all the client connections your application needs, without an excess of unused connections, plus at least 3 more connections to support AWS automation. Before modifying the max_connections parameter setting, we recommend that you consider the following:

  • If the max_connections value is too low, the Aurora PostgreSQL DB instance might not have sufficient connections available when clients attempt to connect. If this happens, attempts to connect using psql raise error messages such as the following: 

    psql: FATAL: remaining connection slots are reserved for non-replication superuser connections

  • If the max_connections value exceeds the number of connections that are actually needed, the unused connections can cause performance to degrade.

The value of max_connections is derived from the following Aurora PostgreSQL LEAST function:

LEAST({DBInstanceClassMemory/9531392},5000)

If you want to change the value for max_connections, you need to create a custom DB cluster parameter group and change its value there. After applying your custom DB parameter group to your cluster, be sure to reboot the primary instance so the new value takes effect. For more information, see Amazon Aurora PostgreSQL parameters and Creating a DB cluster parameter group.

Following, you can find a table that lists the highest value that should ever be used for max_connections for each DB instance class that can be used with Aurora PostgreSQL. If your application needs more connections than the number listed for your DB instance class, consider the following alternatives:

For details about how Aurora Serverless v2 instances handle this parameter, see Parameters that Aurora computes based on Aurora Serverless v2 maximum capacity.

Instance class Default max_connections value
db.x2g.16xlarge 5000
db.x2g.12xlarge 5000
db.x2g.8xlarge 5000
db.x2g.4xlarge 5000
db.x2g.2xlarge 5000
db.x2g.xlarge 5000
db.x2g.large 3479
db.r6g.16xlarge 5000
db.r6g.12xlarge 5000
db.r6g.8xlarge 5000
db.r6g.4xlarge 5000
db.r6g.2xlarge 5000
db.r6g.xlarge 3479
db.r6g.large 1722
db.r5.24xlarge 5000
db.r5.16xlarge 5000
db.r5.12xlarge 5000
db.r5.8xlarge 5000
db.r5.4xlarge 5000
db.r5.2xlarge 5000
db.r5.xlarge 3300
db.r5.large 1600
db.r4.16xlarge 5000
db.r4.8xlarge 5000
db.r4.4xlarge 5000
db.r4.2xlarge 5000
db.r4.xlarge 3200
db.r4.large 1600
db.t4g.large 844
db.t4g.medium 405
db.t3.large 844
db.t3.medium 420

Temporary storage limits for Aurora PostgreSQL

Aurora PostgreSQL stores tables and indexes in the Aurora storage subsystem. Aurora PostgreSQL uses separate temporary storage for non-persistent temporary files. This includes files that are used for such purposes as sorting large data sets during query processing or for index build operations. These local storage volumes are backed by Amazon Elastic Block Store and can be extended by using a larger DB instance class. For more information about storage, see Amazon Aurora storage and reliability.

The following table shows the maximum amount of temporary storage available for each Aurora PostgreSQL DB instance class. For more information on DB instance class support for Aurora, see Aurora DB instance classes.

DB instance class Maximum temporary storage available (GiB)
db.x2g.16xlarge1829
db.x2g.12xlarge1606
db.x2g.8xlarge1071
db.x2g.4xlarge535
db.x2g.2xlarge268
db.x2g.xlarge134
db.x2g.large67
db.r6g.16xlarge 1008
db.r6g.12xlarge 756
db.r6g.8xlarge 504
db.r6g.4xlarge 252
db.r6g.2xlarge 126
db.r6g.xlarge 63
db.r6g.large 32
db.r5.24xlarge 1500
db.r5.16xlarge 1008
db.r5.12xlarge 748
db.r5.8xlarge 504
db.r5.4xlarge 249
db.r5.2xlarge 124
db.r5.xlarge 62
db.r5.large 31
db.r4.16xlarge 960
db.r4.8xlarge 480
db.r4.4xlarge 240
db.r4.2xlarge 120
db.r4.xlarge 60
db.r4.large 30
db.t4g.large 16.5
db.t4g.medium 8.13
db.t3.large 16
db.t3.medium 7.5

You can monitor the temporary storage available for a DB instance with the FreeLocalStorage CloudWatch metric, described in Amazon CloudWatch metrics for Amazon Aurora. (This doesn't apply to Aurora Serverless v2.)

For some workloads, you can reduce the amount of temporary storage by allocating more memory to the processes that are performing the operation. To increase the memory available to an operation, increasing the values of the work_mem or maintenance_work_mem PostgreSQL parameters.