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Database Administration
Maintaining and Monitoring Your Database : Managing Performance : Server performance factors : Before-image I/O : Using a before-image writer
 
Using a before-image writer
The BIW is a background process that continually writes filled BI buffers to disk. Since writes to the BI file occur in the background, client and server processes rarely have to wait for a filled buffer to be written to disk. BIWs are optional, but highly recommended for improving I/O performance.
The server writes current information to the BI file through the current output buffer. When this buffer fills, the server places the buffer on the filled chain. The server then takes a new buffer from the empty chain and uses it as the current output buffer. If no empty buffers are available, the process must wait while a filled buffer is written to disk.
The BIW writes the filled buffers to disk and places them on the empty chain. By clearing out the filled chain, the BIW ensures that a supply of empty buffers is available to client and server processes.
You can only run one BIW per database. You must manually start the BIW, but you can start and stop the BIW process at any time without shutting down the database. See Starting Up and Shutting Down for instructions on starting and stopping a BIW.