Optimizing recovery performance

There are strategies that you can use to improve DB2® performance during database recovery and decrease the time that is required to recover from a DB2 service outage.

The following should be considered when thinking about recovery performance:

• You can improve performance for databases that are frequently updated by placing the logs on a separate device. In the case of an online transaction processing (OLTP) environment, often more I/O is needed to write data to the logs than to store a row of data. Placing the logs on a separate device will minimize the disk arm movement that is required to move between a log and the database files.

  You should also consider what other files are on the disk. For example, moving the logs to the disk used for system paging in a system that has insufficient real memory will defeat your tuning efforts.

  DB2 database products automatically attempt to minimize the time it takes to complete a backup or restore operation by choosing an optimal value for the number of buffers, the buffer size and the parallelism settings. The values are based on the amount of utility heap memory available, the number of processors available and the database configuration.

• To reduce the amount of time required to complete a restore operation, use multiple source devices.
• If a table contains large amounts of long field and LOB data, restoring it could be very time consuming. If the database is enabled for rollforward recovery, the RESTORE command provides the capability to restore selected table spaces. If the long field and LOB data is critical to your business, restoring these table spaces should be considered against the time required to complete the backup task for these table spaces. By storing long field and LOB data in separate table spaces, the time required to complete the restore operation can be reduced by choosing not to restore the table spaces containing the long field and LOB data. If the LOB data can be reproduced from a separate source, choose the NOT LOGGED option when creating or altering a table to include LOB columns. If you choose not to restore the table spaces that contain long field and LOB data, but you need to restore the table spaces that contain the table, you must roll forward to the end of the logs so that all table spaces that contain table data are consistent.
  Note: If you back up a table space that contains table data without the associated long or LOB fields, you cannot perform point-in-time rollforward recovery on that table space. All the table spaces for a table must be rolled forward simultaneously to the same point in time.
• The following apply for both backup and restore operations:
  • Multiple devices should be used.
  • Do not overload the I/O device controller bandwidth.
• DB2 database products use multiple agents to perform both crash recovery and database rollforward recovery. You can expect better performance during these operations, particularly on symmetric multi-processor (SMP) machines; using multiple agents during database recovery takes advantage of the extra CPUs that are available on SMP machines.

  The agent type introduced by parallel recovery is db2agnsc. DB2 database managers choose the number of agents to be used for database recovery based on the number of CPUs on the machine.

  DB2 database managers distribute log records to these agents so that they can be reapplied concurrently, where appropriate. For example, the processing of log records associated with insert, delete, update, add key, and delete key operations can be parallelized in this way. Because the log records are parallelized at the page level (log records on the same data page are processed by the same agent), performance is enhanced, even if all the work was done on one table.

• When you perform a recover operation, DB2 database managers will automatically choose an optimal value for the number of buffers, the buffer size and the parallelism settings. The values will be based on the amount of utility heap memory available, the number of processors available and the database configuration. Therefore, depending on the amount of storage available on your system, you should consider allocating more memory by increasing the util_heap_sz configuration parameter.