The files can be defined to expand automatically by setting the FILEGROWTH value for the log. The transaction log can be implemented in several files. The format of log records and pages is not constrained to follow the format of data pages. For more information, see Transaction Log Physical Architecture.
RAZORSQL LOG CODE
The log cache is managed separately from the buffer cache for data pages, which results in simple, fast, and robust code within the SQL Server Database Engine. The transaction log is implemented as a separate file or set of files in the database. Transaction log characteristicsĬharacteristics of the SQL Server Database Engine transaction log: For more information, see Database Mirroring. The principal server instance sends each log record immediately to the mirror server instance, which applies the incoming log records to the mirror database, continually rolling it forward.
RAZORSQL LOG UPDATE
In a database mirroring scenario, every update to a database, the principal database, is immediately reproduced in a separate, full copy of the database, the mirror database. For more information, see About Log Shipping. Each secondary server restores the log backups to its local secondary database. In a log shipping scenario, the primary server sends the transaction log backups of the primary database to one or more destinations. For more information, see Always On Failover Cluster Instances The primary replica sends each log record immediately to the secondary replicas, that applies the incoming log records to availability group databases, continually rolling it forward. In an Always On availability groups scenario, every update to a database, the primary replica, is immediately reproduced in separate, full copies of the database, the secondary replicas. The standby-server solutions, Always On availability groups, database mirroring, and log shipping, rely heavily on the transaction log. Supporting high availability and disaster recovery solutions For more information, see How Transactional Replication Works. The Log Reader Agent monitors the transaction log of each database configured for transactional replication and copies the transactions marked for replication from the transaction log into the distribution database. For more information, see Restore and Recovery Overview (SQL Server). When the last log backup is restored, the Database Engine then uses the log information to roll back all transactions that were not complete at that point. You first restore the last full database backup and the last differential database backup, and then restore the subsequent sequence of the transaction log backups to the point of failure.Īs you restore each log backup, the Database Engine reapplies all the modifications recorded in the log to roll forward all the transactions. Rolling a restored database, file, filegroup, or page forward to the point of failureĪfter a hardware loss or disk failure affecting the database files, you can restore the database to the point of failure.
Every incomplete transaction found in the transaction log is then rolled back to make sure the integrity of the database is preserved. Every modification recorded in the log that may not have been written to the data files is rolled forward. When an instance of SQL Server is started, it runs a recovery of each database. If a server fails, the databases may be left in a state where some modifications were never written from the buffer cache to the data files, and there may be some modifications from incomplete transactions in the data files. Recovery of all incomplete transactions when SQL Server is started If an application issues a ROLLBACK statement, or if the Database Engine detects an error such as the loss of communication with a client, the log records are used to roll back the modifications made by an incomplete transaction. Supporting high availability and disaster recovery solutions: Always On availability groups, database mirroring, and log shipping.
The transaction log supports the following operations: Operations supported by the transaction log For more information, see Database Checkpoints (SQL Server). Known good points from which to begin applying transaction logs during database recovery are created by checkpoints.
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