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3. Understanding the rsnapshot Configuration

3.1 'Frequency' Labels

docker-mariadb-snapshot's environment variable names and rsnapshot configuration use the terms hourly, daily, weekly, and monthly to refer to different snapshot retention levels:

retain  hourly  RSNAPSHOT_RETAIN_HOURLY
retain  daily   RSNAPSHOT_RETAIN_DAILY
retain  weekly  RSNAPSHOT_RETAIN_WEEKLY
retain  monthly RSNAPSHOT_RETAIN_MONTHLY

Although these appear to be time-based settings, it is important to understand rsnapshot does not inherently understand time. 'hourly', 'daily', 'weekly', and 'monthly' are simply labels for separate snapshot retention groups without any intrinsic time meaning. They could, in fact, be anything - 'red', 'blue', 'green', etc.

The actual frequency of when each level is executed is solely determined by the period in which you execute docker-mariadb-snapshot (e.g., via cron jobs).

3.2 Snapshot Rotation

By default, rsnapshot uses a rotation mechanism to manage snapshots in a hierarchy. In the above-mentioned sample configuration, the daily, weekly, and monthly levels 'promote' and copy/link the oldest snapshot from the level above it instead of creating a new snapshot from live data. Snapshots are only ever taken (by default) at the hourly level, and the other levels simply rotate hourly snapshots down the hierarchy.

This default behavior is not ideal for accurate point-in-time restores. Instead, snapshots should be taken each time the image is run.

To accomplish this, docker-mariadb-snapshot sets the sync_first 1 rsnapshot configuration item, which means running docker-mariadb-snapshot with any frequency level as an argument will always snapshot the live data before performing the rotation, and rotation only happens within the level you invoke.

This method is less efficient in terms of storage and snapshot time, but it guarantees that each snapshot level contains a full snapshot of the live data at the time of execution.

As an example: running docker-mariadb-snapshot with, say, the daily argument will:

  • Create a new snapshot from the live filesystem.
  • Rotate the daily.* set (daily.6 → daily.7, ..., daily.0 → daily.1)
  • Set the new snapshot as daily.0.

It will completely ignore the other levels and never 'promote' snapshots from hourly to daily, daily to weekly, or weekly to monthly.