Common ways to flash redo log into disk in SQL

In the database system,redo log(Redo log) is a critical log file that is used to restore unfinished transactions after a system crash. Willredo logThe method of flashing to disk ensures persistence and consistency of data. Below isredo logCommon methods and procedures for flashing into disks:

Redo Log How to flush the disk

• Write-Ahead Logging (WAL)：

  • in principle: Before writing the actual data to disk, first place the correspondingredo logRecords are written to disk. This ensures that even if the system crashes, it can pass during restartredo logRecover data.

• Buffer mechanism：

  • Redo Log Buffer: The database system maintains aredo log buffer, temporary storageredo logRecord. This buffer can improve the performance of log writing, because writing the log to memory first is faster than writing it directly to disk.

• Log flushing strategy：

  • Periodic brushing: The database system will regularlyredo log bufferThe contents in it are flushed to disk. This can be triggered by timing tasks or reaching a certain size threshold.
  • Flash in when transaction is submitted: When a transaction is submitted, the system will generate the transaction.redo logRecords are flushed to disk synchronously to ensure transaction persistence.
  • Checkpoint: When creating a checkpoint, the system will flush all undiskedredo logRecords are flushed to disk and update checkpoint information. This helps reduce recovery time because logging before the checkpoint is already persisted.

Redo Log flashing process to disk

• Generate Redo Log Records：

  • During transaction execution, the database system will generate the correspondingredo logRecords, These records describe transaction changes to data.

• Write to Redo Log Buffer：

  • Generatedredo logRecords will be written to memory firstredo log buffer。

• Condition triggers flashing into disk：

  • Transaction Submission: When a transaction is submitted, the system will triggerredo logFlash to disk operation. This operation ensures that all change records of the transaction are persisted to disk.
  • The buffer is full:whenredo log bufferWhen a certain threshold is reached, the system will write the log records in the buffer to disk to make room to continue storing new log records.
  • Timely brushing: The database system can configure timing tasks and regularlyredo log bufferThe contents in it are flushed to disk.
  • Checkpoint creation: When creating a checkpoint, the system will flush all undiskedredo logRecords are flushed to disk.

• Flash to disk：

  • Willredo log bufferThe log records in the disk are written to theredo log file. This process is usually achieved through synchronous I/O operations to ensure that logging is available immediately after being written to disk.

Code example (pseudocode)

Below is a pseudo-code example, showingredo logThe process of flushing into disk:

class RedoLogBuffer {
    private List<RedoLogRecord> buffer = new ArrayList<>();
    private final int BUFFER_SIZE = 1024; // Sample size
    public synchronized void addLog(RedoLogRecord log) {
        (log);
        if (() >= BUFFER_SIZE) {
            flushToDisk();
        }
    }

    public synchronized void flushToDisk() {
        if (()) {
            return;
        }
        // Write log records to disk        writeToDisk(buffer);
        ();
    }

    private void writeToDisk(List<RedoLogRecord> logs) {
        // Actual disk write operation        // This is simplified to print logging        for (RedoLogRecord log : logs) {
            ("Writing log to disk: " + log);
        }
    }
}

class RedoLogRecord {
    private String data;

    public RedoLogRecord(String data) {
         = data;
    }

    @Override
    public String toString() {
        return data;
    }
}

class Transaction {
    private RedoLogBuffer redoLogBuffer;

    public Transaction(RedoLogBuffer redoLogBuffer) {
         = redoLogBuffer;
    }

    public void execute(String operation) {
        // Execute the operation and generate the Redo Log        RedoLogRecord log = new RedoLogRecord(operation);
        (log);
    }

    public void commit() {
        // When submitting transactions, make sure all log records are flushed to disk        ();
        ("Transaction committed.");
    }
}

public class RedoLogExample {
    public static void main(String[] args) {
        RedoLogBuffer redoLogBuffer = new RedoLogBuffer();
        Transaction tx = new Transaction(redoLogBuffer);

        ("UPDATE table SET column = 'value' WHERE id = 1");
        ("UPDATE table SET column = 'value' WHERE id = 2");

        // Submit transaction        ();
    }
}

This pseudo-code example shows how to maintain it in memoryredo log buffer, and flush log records to disk if needed. Implementation in actual production environments will be more complex, involving more detailed error handling, synchronization mechanisms and performance optimization.

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