Backup and Recovery - II - Checkpoint - Transaction log – active portion - Database Recovery

Checkpoints A checkpoint causes modified (i.e. dirty) data and log pages from the buffer cache of the current database to be written to disk – this is called flushing This minimizes the number of modifications that have to be rolled forward during a recovery Log file is sequential and every record has a Log Sequence Number (LSN)

Checkpoint Checkpoints occur: when a CHECKPOINT statement is executed (only the current database is checkpointed) when ALTER DATABASE is used to change a database option when SQL Server is stopped by SHUTDOWN statement or by using SQL Server Service Manager to stop running an instance of database

Checkpoint Processes A SQL Server 2000 checkpoint performs these processes in the current database: Writes to the log file a record marking the start of the checkpoint Stores information recorded for the checkpoint in a chain of checkpoint log records. The LSN of the start of this chain is written to the database boot page Records the LSN of the first log image that must be present for a successful database- wide rollback.

Checkpoint Processes (ctd) A SQL Server 2000 checkpoint also performs these processes in the current database: Records a list of all outstanding, active transactions Determines the Minimum Recovery LSN (MinLSN) Deletes all log records before the new MinLSN if using simple recovery model Writes to disk all dirty log and data pages Writes to log file a record marking the end of checkpoint.

Minimum LSN MinLSN is the minimum of: LSN of the start of the checkpoint LSN of the start of oldest active transaction LSN of the start of the oldest replication transaction that has not yet replicated to all subscribers

Active Portion of Trans Log Portion of the log file from the MinLSN to the last-written log record is called the active portion of the log This is the portion of the log required to do a full recovery of the database No part of the active log can ever be truncated All log truncation must be done from the parts of the log before the MinLSN.

MinLSN Example See BOL - active portion of log for illustration of a simplified version of the end of a transaction log with 2 active transactions where: LSN 148 is last record in the transaction log Checkpoint was recorded at LSN 147 At checkpoint, Tran 1 had been committed and Tran 2 was the only active transaction First log record for Tran 2 is oldest log record for active transaction at time of last checkpoint Begin transaction record for Tran 2, LSN 142, is the MinLSN for this checkpoint

Recovery Recovery : ensure data is in a consistent state Each time an instance of SQL Server starts, it recovers each database by rolling back transactions that did not commit and rolling forward transactions that did commit but whose changes were not yet written to disk when an instance of SQL Server stopped use recovery interval database option to set the maximum number of minutes needed to recover database (default is 0 with recovery time under 1 minute and checkpoint about every minute for an active database)

Recovery to a Point in Time You can recover to a point in time by recovering only the transactions that occurred before a specific point in time within a transaction log backup, rather than the entire backup. By viewing the header information of each transaction log backup or the information in the backupset table in msdb, you can quickly identify which backup contains the time to which you want to restore the database. You then need only apply transaction log backups up to that point.

Point in Time Recovery (ctd) Cannot skip specific transactions because any transactions that occur after the transaction you want to undo might depend on the data modified by the undone transaction This would compromise the integrity of the data in the database

Point in Time Recovery (ctd) If you do not want to restore any modifications made to the database after a specific point in time: Restore last database backup without recovering the database Apply each transaction log backup in the same sequence in which they were created Recover the database at the desired point in time within a transaction log backup. This can also be used to restore a database and transaction logs if some log backups created are missing or damaged