Tablespaces

A tablespace is a robust and widely embraced feature in database management systems. It offers a powerful means to enhance the vertical scalability of a database by decoupling the physical and logical modeling of data. Essentially, it serves as a technique for physical database modeling, enabling the efficient distribution of I/O operations across multiple volumes on distinct storage. It thereby optimizes performance through parallel on-disk read/write operations.

In the context of the database industry, tablespaces play a strategic role, particularly when paired with table partitioning, a logical database modeling technique. They prove instrumental in managing large-scale databases and are also used for tasks such as separating tables from indexes or executing temporary operations.

Tablespaces in PostgreSQL have been playing a pivotal role since 2005 (version 8.0), while declarative partitioning was introduced in 2017 (version 10). Consequently, tablespaces are seamlessly integrated into all supported releases of PostgreSQL. Quoting from the PostgreSQL documentation on tablespaces:

By using tablespaces, an administrator can control the disk layout of a PostgreSQL installation. This is useful in at least two ways.

• First, if the partition or volume on which the cluster was initialized runs out of space and cannot be extended, a tablespace can be created on a different partition and used until the system can be reconfigured.
• Second, tablespaces allow an administrator to use knowledge of the usage pattern of database objects to optimize performance.

Declarative tablespaces

CloudNativePG provides support for PostgreSQL tablespaces through declarative tablespaces, operating at two distinct levels:

• Kubernetes, managing persistent volume claims, identically to how PGDATA and WAL volumes are handled
• PostgreSQL, managing the TABLESPACE global objects in the PostgreSQL instance

Being a part of the Kubernetes ecosystem, CloudNativePG's declarative tablespaces are implemented by leveraging persistent volume claims (and persistent volumes). Each tablespace defined in the cluster is housed in its own persistent volume. CloudNativePG takes care of generating the PVCs. It mounts the required volumes in the instance pods in normalized locations and ensures replicas are ready to support tablespaces before activating them in the primary.

You can set up tablespaces when creating the cluster or add them later, provided the storage is available when requested. Currently, you can't remove them. However, this limitation will be addressed in a future minor or patch version of CloudNativePG.

Using declarative tablespaces

Using declarative tablespaces is straightforward. You can find a full example in cluster-example-with-tablespaces.yaml.

To use them, use the new tablespaces stanza on a new or existing Cluster resource:

spec:
  instances: 3

  # ...

  tablespaces:
    - name: tbs1
      storage:
        size: 1Gi
    - name: tbs2
      storage:
        size: 2Gi
    - name: tbs3
      storage:
        size: 2Gi

Each tablespace has its own storage section where you can configure the size and the storage class of the generated PVC. The administrator can thus plan to use different storage classes for different kinds of workloads, as explained in Storage classes and tablespaces.

CloudNativePG creates the persistent volume claims for each instance in the high-availability Postgres cluster. It mounts them in each pod when they have been provisioned. Then, it ensures that the tbs1, tbs2, and tbs3 tablespaces are created on the primary PostgreSQL instance using the CREATE TABLESPACE command. This process is quick, and you see this reflected in Postgres:

app=# SELECT oid, spcname FROM pg_tablespace;
  oid  |      spcname       
-------+--------------------
  1663 | pg_default
  1664 | pg_global
 16387 | tbs1
 16388 | tbs2
 16389 | tbs3
(5 rows)

You can start using them right away:

app=# CREATE TABLE fibonacci(num INTEGER) TABLESPACE tbs1;
CREATE TABLE

The cluster status has a section for tablespaces:

status:

  <- snipped ->
  tablespacesStatus:
  - name: atablespace
    state: reconciled
  - name: another_tablespace
    state: reconciled
  - name: tablespacea1
    state: reconciled

Storage classes and tablespaces

You can use different storage classes for your tablespaces, just as you can for PGDATA and WAL volumes. This is a convenient way of optimizing your resources, balancing performance and costs of your storage based on data access usage and expectations.

This example helps to explain the feature:

apiVersion: postgresql.cnpg.io/v1
kind: Cluster
metadata:
  name: yardbirds
spec:
  instances: 3

  storage:
    size: 10Gi
  walStorage:
    size: 10Gi
  tablespaces:
    - name: current
      size: 100Gi
      storageClass: fastest
    - name: this_year
      size: 500Gi
      storageClass: balanced

The yardbirds cluster example requests 4 persistent volume claims using 3 different storage classes:

• Default storage class – Used by the PGDATA and WAL volumes.
• fastest – Used by the current tablespace to store the most active and demanding set of data in the database.
• balanced – Used by the this_year tablespace to store older partitions of data that are rarely accessed by users and where performance expectations aren't the highest.

You can then take advantage of horizontal table partitioning and create the current month's table (for example, facts for December 2023) in the current tablespace:

CREATE TABLE facts_202312 PARTITION OF facts
    FOR VALUES FROM ('2023-12-01') TO ('2024-01-01')
    TABLESPACE current;

Tablespace ownership

By default, unless otherwise specified, tablespaces are owned by the app application user, as defined in .spec.bootstrap.initdb.owner. See Bootstrap a new cluster for details. This default behavior works in most microservice database use cases.

You can set the owner of a tablespace in the owner stanza, for example the postgres user, like in the following excerpt:

  # ...
  tablespaces:
    - name: clapton
      owner:
        name: postgres
      storage:
        size: 1Gi

If you define a tablespace with an owner that doesn't exist, CloudNativePG can't create the tablespace and reflects this in the cluster status:

spec:
  instances: 3

  # ...

  tablespaces:
    - name: tbs1
      storage:
        size: 1Gi
    - name: tbs2
      storage:
        size: 2Gi
    - name: tbs3
      owner:
        name: badhombre
      storage:
        size: 2Gi
        status:

  <- snipped ->
  tablespacesStatus:
  - name: tbs1
    status: reconciled
  - name: tbs2
    status: reconciled
  - error: 'while creating tablespace tbs3: ERROR: role "badhombre" does
      not exist (SQLSTATE 42704)'
    name: tbs3
    status: pending

Backup and recovery

CloudNativePG handles backup of tablespaces (and the relative tablespace map) both on object stores and volume snapshots.

Once a cluster with tablespaces has a base backup, you can restore a new cluster from it. When it comes to the recovery side, it's your responsibility to ensure that the Cluster definition of the recovered database contains the exact list of tablespaces.

Replica clusters

Replica clusters must have the same tablespace definition as their origin. The reason is that tablespace management commands like CREATE TABLESPACE are WAL logged and are replayed by any physical replication client (streaming or by way of WAL shipping).

It's your responsibility to ensure that replica clusters have the same list of tablespaces, with the same name. Storage class and size might vary.

For example:

spec:

  # ...
  bootstrap:
    recovery:
      # ... your selected recovery method

  tablespaces:
    - name: tbs1
      storage:
        size: 1Gi
    - name: tbs2
      storage:
        size: 2Gi
    - name: tbs3
      storage:
        size: 2Gi

Temporary tablespaces

PostgreSQL allows you to define one or more temporary tablespaces to create temporary objects (temporary tables and indexes on temporary tables) when a CREATE command doesn't explicitly specify a tablespace, and to create temporary files for purposes such as sorting large data sets. When no temporary tablespace is specified, PostgreSQL uses the default tablespace of a database, which is currently the main PGDATA volume.

When you specify more than one temporary tablespace, PostgreSQL randomly picks one the first time a temporary object needs to be created in a transaction. Then it sequentially iterates through the list.

Temporary tablespaces also work like regular tablespaces with regard to backups.

CloudNativePG provides the .spec.tablespaces[*].temporary option to determine whether to add a tablespace to the temp_tablespaces PostgreSQL parameter and thus become eligible to store temporary data that doesn't have an explicit tablespace assignment.

spec:
  [...]
  tablespaces:
    - name: atablespace
      storage:
        size: 1Gi
      temporary: true

They can be created at initialization time or added later, requiring a rolling update. The temporary: true/false option adds or removes the tablespace name to or from the list of tablespaces in the temp_tablespaces option. This change doesn't require a restart of PostgreSQL.

Although temporary tablespaces can also work as regular tablespaces (meaning that users can also host regular data on them while using them for temporary operations), we recommend that you don't mix the two workloads.

See the PostgreSQL documentation on temp_tablespaces for details.

kubectl plugin support

The kubectl status plugin includes a section dedicated to tablespaces that offers a convenient overview, including tablespace status, owner, temporary flag, and any errors:

[...]

Tablespaces status
Tablespace          Owner  Status      Temporary  Error
----------          -----  ------      ---------  -----
atablespace         app    reconciled  true       
another_tablespace  app    reconciled  true       
tablespacea1        app    reconciled  false 

Instances status
[...]

Limitations

Currently, you can't remove tablespaces from an existing CloudNativePG cluster.