Handling «Disk Full» situations

Last updated January 23rd, 2023

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 mail 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:

• Redis, Kafka MirrorMaker, Grafana, M3 Aggregator and Kafka Connect do not store any user data on disk. Thus they will not fill up the underlying disk storage.
• Kafka, OpenSearch, Cassandra and M3DB turn to read-only.
• MySQL and PostgreSQL turn to read-only with a way to temporarily revert to read-write.
• MongoDB forbids writes but allows deletes.

What to do: 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.

What to do: Reclaim disk space

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

Kafka, OpenSearch, M3DB

You can reclaim disk space by deleting a Kafka topic, an OpenSearch index or an M3DB namespace.

MongoDB

MongoDB refuses any query that inserts data, but allows queries deleting data. You can thus execute any MongoDB command that allows 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.

We want your feedback!

We would love to help answer questions and appreciate any feedback you may have.

Are you on Discord? Connect to our channel at https://discord.gg/ovhcloud and interact directly with the team that builds our databases service!

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