Limits in Azure Database for PostgreSQL - Flexible Server

APPLIES TO: Azure Database for PostgreSQL - Flexible Server

The following sections describe capacity and functional limits in Azure Database for PostgreSQL flexible server. If you'd like to learn about resource (compute, memory, storage) tiers, see the compute and storage article.

Maximum connections

Below, you'll find the default maximum number of connections for each pricing tier and vCore configuration. Please note, Azure Database for PostgreSQL flexible server reserves 15 connections for physical replication and monitoring of the Azure Database for PostgreSQL flexible server instance. Consequently, the max user connections value listed in the table is reduced by 15 from the total max connections.

SKU Name vCores Memory Size Max Connections Max User Connections
Burstable
B1ms 1 2 GiB 50 35
B2s 2 4 GiB 429 414
B2ms 2 8 GiB 859 844
B4ms 4 16 GiB 1718 1703
B8ms 8 32 GiB 3437 3422
B12ms 12 48 GiB 5000 4985
B16ms 16 64 GiB 5000 4985
B20ms 20 80 GiB 5000 4985
General Purpose
D2s_v3 / D2ds_v4 / D2ds_v5 / D2ads_v5 2 8 GiB 859 844
D4s_v3 / D4ds_v4 / D4ds_v5 / D4ads_v5 4 16 GiB 1718 1703
D8s_v3 / D8ds_V4 / D8ds_v5 / D8ads_v5 8 32 GiB 3437 3422
D16s_v3 / D16ds_v4 / D16ds_v5 / D16ads_v5 16 64 GiB 5000 4985
D32s_v3 / D32ds_v4 / D32ds_v5 / D32ads_v5 32 128 GiB 5000 4985
D48s_v3 / D48ds_v4 / D48ds_v5 / D48ads_v5 48 192 GiB 5000 4985
D64s_v3 / D64ds_v4 / D64ds_v5 / D64ads_v5 64 256 GiB 5000 4985
D96ds_v5 / D96ads_v5 96 384 GiB 5000 4985
Memory Optimized
E2s_v3 / E2ds_v4 / E2ds_v5 / E2ads_v5 2 16 GiB 1718 1703
E4s_v3 / E4ds_v4 / E4ds_v5 / E4ads_v5 4 32 GiB 3437 3422
E8s_v3 / E8ds_v4 / E8ds_v5 / E8ads_v5 8 64 GiB 5000 4985
E16s_v3 / E16ds_v4 / E16ds_v5 / E16ads_v5 16 128 GiB 5000 4985
E20ds_v4 / E20ds_v5 / E20ads_v5 20 160 GiB 5000 4985
E32s_v3 / E32ds_v4 / E32ds_v5 / E32ads_v5 32 256 GiB 5000 4985
E48s_v3 / E48ds_v4 / E48ds_v5 / E48ads_v5 48 384 GiB 5000 4985
E64s_v3 / E64ds_v4 / E64ds_v5 / E64ads_v5 64 432 GiB 5000 4985
E96ds_v5 / E96ads_v5 96 672 GiB 5000 4985

Note

The reserved connection slots, presently at 15, could change. We advise regularly verifying the total reserved connections on the server. This is calculated by summing the values of 'reserved_connections' and 'superuser_reserved_connections' server parameters. The maximum available user connections is max_connections - (reserved_connections + superuser_reserved_connections).

Note

That default value for the max_connections server parameter is calculated when the instance of Azure Database for PostgreSQL Flexible Server is first provisioned, based on the SKU name selected for its compute. Any subsequent changes of SKU to the compute supporting that flexible server, won't have any effect on the currently set neither on the default value chosen for max_connections server parameter of that instance. Therefore it is recommended that, whenever you change the SKU assigned to an instance, you also adjust the currently set value for the max_connections parameter as per the values provided in the table above.

Changing the max_connections value

When you first set up your Azure Postgres Flexible Server, it automatically decides the highest number of connections it can handle concurrently. This number is based on your server's configuration and cannot be changed.

However, you can adjust how many connections are allowed at any given time. To do this, change the 'max_connections' setting. Remember, after you change this setting, you'll need to restart your server for the new limit to start working.

Caution

While it is possible to increase the value of max_connections beyond the default setting, it is not advisable. The rationale behind this recommendation is that instances may encounter difficulties when the workload expands and demands more memory. As the number of connections increases, memory usage also rises. Instances with limited memory may face issues such as crashes or high latency. Although a higher value for max_connections might be acceptable when most connections are idle, it can lead to significant performance problems once they become active. Instead, if you require additional connections, we suggest utilizing pgBouncer, Azure's built-in connection pool management solution, in transaction mode. To start, it is recommended to use conservative values by multiplying the vCores within the range of 2 to 5. Afterward, carefully monitor resource utilization and application performance to ensure smooth operation. For detailed information on pgBouncer, please refer to the PgBouncer in Azure Database for PostgreSQL - Flexible Server.

When connections exceed the limit, you may receive the following error:

FATAL: sorry, too many clients already.

When using Azure Database for PostgreSQL flexible server for a busy database with a large number of concurrent connections, there may be a significant strain on resources. This strain can result in high CPU utilization, particularly when many connections are established simultaneously and when connections have short durations (less than 60 seconds). These factors can negatively impact overall database performance by increasing the time spent on processing connections and disconnections. It's important to note that each connection in Azure Database for PostgreSQL flexible server, regardless of whether it is idle or active, consumes a significant amount of resources from your database. This consumption can lead to performance issues beyond high CPU utilization, such as disk and lock contention. The topic is discussed in more detail in the PostgreSQL Wiki article on the Number of Database Connections. To learn more, visit Identify and solve connection performance in Azure Database for PostgreSQL flexible server.

Functional limitations

Scale operations

  • At this time, scaling up the server storage requires a server restart.
  • Server storage can only be scaled in 2x increments, see Compute and Storage for details.
  • Decreasing server storage size is currently not supported. The only way to do is dump and restore it to a new Azure Database for PostgreSQL flexible server instance.

Storage

  • Once configured, storage size can't be reduced. You have to create a new server with desired storage size, perform manual dump and restore and migrate your database(s) to the new server.
  • When the storage usage reaches 95% or if the available capacity is less than 5 GiB whichever is more, the server is automatically switched to read-only mode to avoid errors associated with disk-full situations. In rare cases, if the rate of data growth outpaces the time it takes to switch to read-only mode, your Server may still run out of storage. You can enable storage autogrow to avoid these issues and automatically scale your storage based on your workload demands.
  • We recommend setting alert rules for storage used or storage percent when they exceed certain thresholds so that you can proactively take action such as increasing the storage size. For example, you can set an alert if the storage percentage exceeds 80% usage.
  • If you're using logical replication, then you must drop the logical replication slot in the primary server if the corresponding subscriber no longer exists. Otherwise, the WAL files accumulate in the primary filling up the storage. If the storage threshold exceeds certain threshold and if the logical replication slot isn't in use (due to a non-available subscriber), Azure Database for PostgreSQL flexible server automatically drops that unused logical replication slot. That action releases accumulated WAL files and avoids your server becoming unavailable due to storage getting filled situation.
  • We don't support the creation of tablespaces, so if you're creating a database, don’t provide a tablespace name. Azure Database for PostgreSQL flexible server uses the default one that is inherited from the template database. It's unsafe to provide a tablespace like the temporary one because we can't ensure that such objects will remain persistent after server restarts, HA failovers, etc.

Networking

  • Moving in and out of VNET is currently not supported.
  • Combining public access with deployment within a VNET is currently not supported.
  • Firewall rules aren't supported on VNET, Network security groups can be used instead.
  • Public access database servers can connect to the public internet, for example through postgres_fdw, and this access can't be restricted. VNET-based servers can have restricted outbound access using Network Security Groups.

High availability (HA)

Availability zones

  • Manually moving servers to a different availability zone is currently not supported. However, using the preferred AZ as the standby zone, you can enable HA. Once established, you can fail over to the standby and then disable HA.

Postgres engine, extensions, and PgBouncer

  • Postgres 10 and older aren't supported as those are already retired by the open-source community. If you must use one of these versions, you need to use the Azure Database for PostgreSQL single server option, which supports the older major versions 9.5, 9.6 and 10.
  • Azure Database for PostgreSQL flexible server supports all contrib extensions and more. Please refer to PostgreSQL extensions.
  • Built-in PgBouncer connection pooler is currently not available for Burstable servers.

Stop/start operation

  • Once you stop the Azure Database for PostgreSQL flexible server instance, it automatically starts after 7 days.

Scheduled maintenance

  • You can change custom maintenance window to any day/time of the week. However, any changes made after receiving the maintenance notification will have no impact on the next maintenance. Changes only take effect with the following monthly scheduled maintenance.

Backing up a server

  • Backups are managed by the system, there's currently no way to run these backups manually. We recommend using pg_dump instead.
  • The first snapshot is a full backup and consecutive snapshots are differential backups. The differential backups only back up the changed data since the last snapshot backup. For example, if the size of your database is 40 GB and your provisioned storage is 64 GB, the first snapshot backup will be 40 GB. Now, if you change 4 GB of data, then the next differential snapshot backup size will only be 4 GB. The transaction logs (write ahead logs - WAL) are separate from the full/differential backups, and are archived continuously.

Restoring a server

  • When using the Point-in-time-Restore feature, the new server is created with the same compute and storage configurations as the server it is based on.
  • VNET based database servers are restored into the same VNET when you restore from a backup.
  • The new server created during a restore doesn't have the firewall rules that existed on the original server. Firewall rules need to be created separately for the new server.
  • Restore to a different subscription isn't supported but as a workaround, you can restore the server within the same subscription and then migrate the restored server to a different subscription.

Next steps