Oracle duplicate RAM caches Oracle Tips by Burleson Consulting

The Problem of Duplicitous RAM Caches

As hardware evolved though the 1990’s, independent components of database systems started to employ their own RAM caching tools as shown in Figure 14.9.

In this figure, the Oracle database is not the only component to utilize RAM caching.  The disk array employs a RAM cache, the servers have a Journal File System (JFS) RAM cache, and the front-end web server also serves to cache Oracle data.

This concept is important because many enterprises may inadvertently double cache Oracle data.  Even more problematic are the fake statistics reported by Oracle when multiple level caches are employed:

§       Fake Physical I/O times: If a disk array with a built-in RAM cache is in use, the disk I/O subsystem may acknowledge (“ack”) a physical write to Oracle, when in reality the data has not yet been written to the physical disk spindle.  This “false ack” can skew timing of disk read/write speeds.

§       Wasted Oracle Data Buffer RAM: In system that employs web servers, the Apache front end may cache frequently used data.  Thus, significant Oracle resources may be wasted by caching data blocks that are already cached on the web server tier.

The next step is to take a look at the best way to use SSD in an Oracle environment.  First is the examination of the relationship between physical disk I/O (PIO) and Oracle Logical I/O (LIO).

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