Physical-Log Buffer

The database server uses the physical-log buffer to hold before-images of some of the modified dbspace pages. The before-images in the physical log and the logical-log records enable the database server to restore consistency to its databases after a system failure.

The physical-log buffer is actually two buffers. Double buffering permits the database server processes to write to the active physical-log buffer while the other buffer is being flushed to the physical log on disk. For a description of how the database server flushes the physical-log buffer, refer to Flushing the Physical-Log Buffer. For information on monitoring the physical-log file, refer to Monitoring Physical and Logical Logging Activity.

The PHYSBUFF parameter in the ONCONFIG file specifies the size of the physical-log buffers. A write to the physical-log buffer writes exactly one page. If the specified size of the physical-log buffer is not evenly divisible by the page size, the database server rounds the size down to the nearest value that is evenly divisible by the page size. Although some operations require the buffer to be flushed sooner, in general the database server flushes the buffer to the physical-log file on disk when the buffer fills. Thus, the size of the buffer determines how frequently the database server needs to flush it to disk. For more information on this configuration parameter, refer to the IBM Informix: Administrator's Reference.