How can I force immediate MDF file updates in a SQL Server Docker container on AWS?

Below is an overview addressing both the SQL Server behavior and Docker volume mapping in your scenario:

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Docker Volume Mapping

• Mapping at Container Start:
  When you run your SQL Server container with a volume mapping (using the -v flag or Docker Desktop configuration), the host directory (or volume) is mounted immediately when the container starts. This means the container’s filesystem—including the location where the .mdf and .ldf files reside—is directly linked to the specified host storage.

• Real-Time Writes:
  SQL Server writes to its data and log files as part of its normal operations. With a Docker volume, these writes are directed to the volume’s storage—provided the volume is correctly mounted, there’s no inherent delay waiting for container shutdown. Any caching or delayed appearance of writes on the host is due to SQL Server’s own buffering mechanisms, not Docker’s volume mapping.

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SQL Server File Update Behavior

• Recovery Modes and Write Patterns:
  As you noted from your research, when the recovery mode is set to FULL, SQL Server primarily logs transactions in the .ldf file. Data pages in the .mdf file might be held in memory and only flushed to disk during checkpoints. This can make it appear that only the log file is updating with actions like user registration.

• Impact of Changing Recovery Mode:
  Switching the recovery mode to SIMPLE means that SQL Server will perform more frequent checkpoints with less intensive logging, which could result in more immediate writes to the .mdf file. If your backup and restore strategy doesn’t require point-in-time recovery, this may be acceptable. However, be cautious because this changes the way you can recover data in the event of a failure.

• Internal Buffering vs. Docker Behavior:
  The observation that the .mdf file only appears to be updated after container shutdown (as mentioned in the Reddit post) is more about SQL Server’s internal buffering or file system caching, not the Docker volume mapping itself. Once the volume is mounted, any changes are written to the host storage; the timing of when you see the changes may vary based on SQL Server’s checkpoint schedules or OS-level caching.

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Recommendations

• Volume Mapping:
  Ensure your volume is attached at container startup. This guarantees that SQL Server writes its data directly to that volume, and the persistence is managed independently of the container lifecycle.

• Recovery Mode Decision:

  • If point-in-time recovery is not essential for your application, switching to SIMPLE recovery mode may help reduce the size and frequency of log file updates.
  • Make sure to thoroughly test behavior under your application’s load, as SQL Server’s flush/commit behavior might still cause some delays in visible file updates—even though the data is being persisted as expected.

• Monitoring File Changes:
  If you’re monitoring file updates on the host, consider that operating system caching and SQL Server’s internal buffering might delay long-term file size changes. Tools that monitor I/O at the file system level might offer better insight than simply checking file modification times.

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Summary

• Docker’s Role:
  Docker volumes are mounted when the container starts, so any SQL Server file writes occur directly on the host’s mapped storage. There is no Docker-induced delay in updating the volume files.

• SQL Server’s Behavior:
  What you observe regarding the .mdf file is likely a result of SQL Server’s checkpoint and caching mechanisms—not that Docker only transfers the file upon shutdown.

• Next Steps:
  Consider carefully whether switching recovery modes aligns with your backup and disaster recovery needs. The volume mapping itself is not at fault for any perceived delay; it’s a matter of how and when SQL Server flushes its internal buffers to disk.

By keeping these points in mind, you can better design your container and database configuration to match your performance and persistence requirements.