Handle 'Disk Full' situations for Public Cloud Databases

Objective

No matter the database technology, when no more physical disk space is available the service operation degrades significantly. At the very least your databases cannot store any more data, but even logical read operations might start to get impacted, for example querying might slow down or fail.

This guide helps you understand how Public Cloud Databases services behave before and when reaching such conditions, and what you can do about it.

Requirements

• Access to the OVHcloud Control Panel
• A Public Cloud database service up and running

Instructions

Avoiding full disk conditions

Disk space usage metrics

As part of using your Public Cloud Databases service efficiently, you should keep an eye on the service metrics. You can access those in the OVHcloud Control Panel or using the API. You can also make use of cross-service integrations to gather, observe and alert based on services metrics.

Mail notifications

When your service storage begins to fill up and reaches a high mark, Public Cloud Databases sends you an email to warn you of the situation. The specific threshold depends on the engine, it may range from 75 to 90 percent.

When the disk usage increases even more and reaches a critical level (depending on the engine, ranging from 90 to 95 percent), you will receive another email notification and the service will turn to a "disk full" mode, where it will start to refuse writes.

How to handle a disk full situation ?

Different engines react in different ways, thus Public Cloud Databases services react differently when facing disk full conditions:

• Valkey does not store any user data on disk. Thus, it will not fill up the underlying disk storage.
• MySQL and PostgreSQL turn to read-only with a way to temporarily revert to read-write.
• MongoDB forbids writes but allows deletes only for the Admin users.

Upgrading your service

It may be that your usage simply requires more storage. You can then increase the provisioned storage, and / or upgrade to an offer with more storage. Once the upgrade finishes, the service will detect that more storage is available and thus revert to normal mode.

Reclaim disk space

It may be that you have reached the full disk situation because of a runaway application filling up your database, or that you are storing some old obsolete data. In these cases, stop whatever process which is unduly filling up your storage, then remove unwanted data.

MongoDB

MongoDB blocks all write operations when the disk is full, except for the Admin users who can execute any MongoDB command to reclaim disk space.

PostgreSQL, MySQL

For these engines, call the respective API endpoint to temporarily allow write operations:

This will give you a 15 minutes time window to write again to your database. At the end of this window, either you were able to execute queries that reduce disk usage (e.g. DROPs, DELETEs), and thus your service changes to the read-write state, or disk usage stays too high and your service will return to the read-only state.

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