:book: Add the database use case for in-place updates

[gbartolini]

As a maintainer of the CloudNativePG project (CNCF Sandbox) and a contributor to PostgreSQL, I have observed a growing demand for database workloads on bare metal within Kubernetes that use local storage on the worker nodes. High Availability and Disaster Recovery depend on application-level replication—specifically, PostgreSQL—for state synchronisation across multiple locations.

This use case would greatly benefit from the ability to perform in-place upgrades of the worker nodes, provided that the database volumes are not deleted. For instance, consider a 5-terabyte database stored on a local volume that is lost during a regular Cluster API upgrade of Kubernetes. In such cases, it may be necessary to restore the database from a backup (for single-instance clusters) or to re-clone it from the primary database, which could take hours. However, if the data volumes were left intact, the PostgreSQL instance would simply resume normal operation after the reboot.

This PR adds a section in the "Motivations" section of the proposal for in-place updates.

/area upgrades

[linux-foundation-easycla[bot]]

• :white_check_mark:login: gbartolini / (378378319f2edfe66f3b4fbd50c5a794b9422460)

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[fabriziopandini]

I'm ok in adding this use case but it would be nice if authors of the proposal can chime in @g-gaston @alexander-demicev

[fabriziopandini]

/ok-to-test

[k8s-ci-robot]

LGTM label has been added.

Git tree hash: 5f4f67a8966ddd3afc92481fe33c5ae1ae53b87a

[sbueringer]

Sounds good.

Please take a look at the cla: https://github.com/kubernetes-sigs/cluster-api/pull/12403#issuecomment-3005512076

We can't merge without it

[gbartolini]

Sounds good.

Please take a look at the cla: #12403 (comment)

We can't merge without it

Done, thanks!

[sbueringer]

Thank you!

/approve

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