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Checkpoints and the Active Portion of the Log

Checkpoints flush dirty data pages from the buffer cache of the current database to disk. This minimizes the active portion of the log that must be processed during a full recovery of a database. During a full recovery, the following types of actions are performed:

  • The log records of modifications not flushed to disk before the system stopped are rolled forward.
  • All modifications associated with incomplete transactions, such as transactions for which there is no COMMIT or ROLLBACK log record, are rolled back.

A checkpoint performs the following processes in the database:

  • Writes a record to the log file, marking the start of the checkpoint.
  • Stores information recorded for the checkpoint in a chain of checkpoint log records.
    One piece of information recorded in the checkpoint is the log sequence number (LSN) of the first log record that must be present for a successful database-wide rollback. This LSN is called the Minimum Recovery LSN (MinLSN). The MinLSN is the minimum of the:
    • LSN of the start of the checkpoint.
    • LSN of the start of the oldest active transaction.
    • LSN of the start of the oldest replication transaction that has not yet been delivered to the distribution database.
    The checkpoint records also contain a list of all the active transactions that have modified the database.
  • If the database uses the simple recovery model, marks for reuse the space that precedes the MinLSN.
  • Writes all dirty log and data pages to disk.
  • Writes a record marking the end of the checkpoint to the log file.
  • Writes the LSN of the start of this chain to the database boot page.

Activities That Cause a Checkpoint

Checkpoints occur in the following situations:

  • A CHECKPOINT statement is explicitly executed. A checkpoint occurs in the current database for the connection.
  • A minimally logged operation is performed in the database; for example, a bulk-copy operation is performed on a database that is using the Bulk-Logged recovery model.
  • Database files have been added or removed by using ALTER DATABASE.
  • An instance of SQL Server is stopped by a SHUTDOWN statement or by stopping the SQL Server (MSSQLSERVER) service. Either action causes a checkpoint in each database in the instance of SQL Server.
  • An instance of SQL Server periodically generates automatic checkpoints in each database to reduce the time that the instance would take to recover the database.
  • A database backup is taken.
  • An activity requiring a database shutdown is performed. For example, AUTO_CLOSE is ON and the last user connection to the database is closed, or a database option change is made that requires a restart of the database.

The SQL Server Database Engine generates automatic checkpoints. The interval between automatic checkpoints is based on the amount of log space used and the time elapsed since the last checkpoint. The time interval between automatic checkpoints can be highly variable and long, if few modifications are made in the database. Automatic checkpoints can also occur frequently if lots of data is modified.

Use the recovery interval server configuration option to calculate the interval between automatic checkpoints for all the databases on a server instance. This option specifies the maximum time the Database Engine should use to recover a database during a system restart. The Database Engine estimates how many log records it can process in the recovery interval during a recovery operation.

The interval between automatic checkpoints also depends on the recovery model:

  • If the database is using either the full or bulk-logged recovery model, an automatic checkpoint is generated whenever the number of log records reaches the number the Database Engine estimates it can process during the time specified in the recovery interval option.
  • If the database is using the simple recovery model, an automatic checkpoint is generated whenever the number of log records reaches the lesser of these two values:
    • The log becomes 70 percent full.
    • The number of log records reaches the number the Database Engine estimates it can process during the time specified in the recovery interval option.

For information about setting the recovery interval, see How to: Set the Recovery Interval (SQL Server Management Studio).

Automatic checkpoints truncate the unused section of the transaction log if the database is using the simple recovery model. However, if the database is using the full or bulk-logged recovery models, the log is not truncated by automatic checkpoints. For more information, see Transaction Log Truncation.

The CHECKPOINT statement now provides an optional checkpoint_duration argument that specifies the requested period of time, in seconds, for checkpoints to finish. For more information, see CHECKPOINT (Transact-SQL).

The section of the log file from the MinLSN to the last-written log record is called the active portion of the log, or the active log. This is the section of the log required to do a full recovery of the database. No part of the active log can ever be truncated. All log records must be truncated from the parts of the log before the MinLSN.

The following illustration shows a simplified version of the end-of-a-transaction log with two active transactions. Checkpoint records have been compacted to a single record.

end-of-a-transaction log with active transactions

LSN 148 is the last record in the transaction log. At the time that the recorded checkpoint at LSN 147 was processed, Tran 1 had been committed and Tran 2 was the only active transaction. That makes the first log record for Tran 2 the oldest log record for a transaction active at the time of the last checkpoint. This makes LSN 142, the Begin transaction record for Tran 2, the MinLSN.

The active log must include every part of all uncommitted transactions. An application that starts a transaction and does not commit it or roll it back prevents the Database Engine from advancing the MinLSN. This can cause two types of problems:

  • If the system is shut down after the transaction has performed many uncommitted modifications, the recovery phase of the subsequent restart can take much longer than the time specified in the recovery interval option.
  • The log might grow very large, because the log cannot be truncated past the MinLSN. This occurs even if the database is using the simple recovery model, in which the transaction log is generally truncated on each automatic checkpoint.

The Log Reader Agent monitors the transaction log of each database configured for transactional replication, and it copies the transactions marked for replication from the transaction log into the distribution database. The active log must contain all transactions that are marked for replication, but that have not yet been delivered to the distribution database. If these transactions are not replicated in a timely manner, they can prevent the truncation of the log. For more information, see How Transactional Replication Works.

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