How to Read Buffer Cache Advisory Section in AWR and Statspack Reports.
(Doc ID 754639.1)
Last updated on OCTOBER 12, 2021
Applies to:Gen 1 Exadata Cloud at Customer (Oracle Exadata Database Cloud Machine) - Version N/A and later
Oracle Cloud Infrastructure - Database Service - Version N/A and later
Oracle Database Backup Service - Version N/A and later
Oracle Database Exadata Express Cloud Service - Version N/A and later
Oracle Database Cloud Exadata Service - Version N/A and later
Information in this document applies to any platform.
One widespread point of view is that in order to lessen the amount of physical I/O, the database buffer cache size should be increased as much as physical memory on the box for this database. That however does not take into account the overhead of serving the bigger buffer cache (at Oracle level, longer search for a required buffer in a much bigger space, in particular), which may outweigh benefits of further reduction in the amount of physical reads, especially if such a reduction is small compared with the portion of the increased buffer cache. Finding the right balance is a difficult task. Buffer Cache Advisory section is a way of addressing this task, as it shows correspondence between the size of the buffer cache and estimated number of physical reads in the context of prospective variations of this size up and down. As seen in this document, interpreting data in this section is simplified with use of two columns 'Size Factor' and 'Estimated Physical Read Factor'.
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