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Oracle Database Tips by Donald Burleson


The Problem of Duplicate 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 1.7.

In this figure, it is clear that the Oracle database is not the only component to utilize RAM caching.  The disk array employs a RAM cache, the servers has a Journal File System(JFS) RAM cache, and the front end web server also serve 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: When using a disk array with a built-in RAM cache, the disk I/O subsystem may acknowledge a physical write to Oracle when, in reality, the data has not yet been written to the physical disk spindle.  This can skew timing of disk read/write speed.

  • Wasted Oracle Data Buffer RAM: In systems that employ web servers, the Apache front end may cache frequently used data.  Hence, significant Oracle resources may be wasted caching data blocks that are already cached on the web server tier.

The next section will cover the best way to use SSD in an Oracle RAC environment.  The information begins with an examination of the relationship between physical disk I/O (POI) and Oracle Logical I/O (LIO).

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This is an excerpt from the book Oracle RAC & Tuning with Solid State Disk.  You can get it for more than 30% by buying it directly from the publisher and get immediate access to working code examples.

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