1 year ago
<@U05TW21LEEN> Is my statement correct? Please give your comment
Depending on the timing of a failure relative to the execution of a checkpoint, the volume of WAL data required for recovery can vary significantly. Let’s consider two scenarios:
Scenario 1: Failure immediately after a checkpoint completion When a failure occurs right after a successful checkpoint, the system potentially has the smallest amount of WAL data needed for recovery. This is because most of the data is already committed (written to disk) during the checkpoint.
Possible volume of WAL data for recovery: It can be very small, as the data is already flushed to disk. In this case, only those WAL files generated after the last checkpoint until the moment of failure might need to be restored. This could amount to just a few gigabytes, depending on the activity in the system during this short period.
Scenario 2: Failure just before a scheduled checkpoint If a failure occurs just before a checkpoint was due to be executed, significantly more data may need to be restored. Since the checkpoint was not completed, all changes made since the last checkpoint must be recovered.
Possible volume of WAL data for recovery: In this case, up to twice the max_wal_size may be required if there was sufficient transactional activity to fill the WAL. This means that if max_wal_size is set to 64GB, up to 128GB of WAL files might be needed for a full data recovery in extreme cases.
Postgres.AI
1 year ago
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Your statement is generally accurate and reflects well the relationship between checkpoints, WAL (Write-Ahead Logging) activity, and the implications for recovery in Postgres.
Here are some detailed comments:
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Checkpoint Mechanics: Checkpoints in Postgres ensure that all data changes (logged in WAL files) up to the point of the checkpoint are written to the database's data files on disk. This process reduces the amount of WAL data needed for recovery because it means that the database can start replaying WAL from the last completed checkpoint rather than from some earlier point in time.
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Scenario 1 (Failure after a checkpoint): Your description here is correct. If a failure occurs right after a checkpoint has successfully completed, the volume of WAL needed for recovery would be minimal, consisting only of the transactions that occurred after that last checkpoint.
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Scenario 2 (Failure before a checkpoint): This scenario is accurately described as well. The volume of WAL needed for recovery can be significantly higher if a failure occurs before a scheduled checkpoint. The max_wal_size setting controls the maximum size of WAL files held between checkpoints. Your example involving up to twice max_wal_size is plausible but represents an extreme case where a significant amount of WAL has been generated and not yet trimmed due to the failure occurring just before a checkpoint.
The exact volume of WAL needed for recovery can vary based on transactional activity, the size of the transactions, and the checkpoint settings. Remember, these scenarios assume ideal conditions and may vary in practice. It's also important to regularly review and adjust the configuration settings like max_wal_size based on the system's workload to strike a balance between performance and recovery considerations.
It would be beneficial to test these scenarios in a controlled environment specific to your workload to understand better the actual impact on your system.
For more detailed information on checkpoints and WAL, refer to the .