Before-imaging is always enabled to let the database engine recover transactions if the system fails. This mechanism is extremely important for database reliability, but it creates a significant amount of I/O that can affect performance. In addition, before-image I/O is usually the first and most likely cause of I/O bottlenecks. The engine must always record a change in the BI file before it can record a change in the database and after-image files. If BI activity creates an I/O bottleneck, all other database activities are affected.

You can reduce the I/O impact of before-imaging by:

  • Moving the BI file to its own disk
  • Running a before-image writer (BIW) on systems with and Enterprise database license
  • Providing more BI buffers
  • Increasing the BI cluster size
  • Increasing the BI block size
  • Delaying BI writes