Oracle Tips by Burleson Consulting

The UNIX buffer cache

Just as Oracle has data buffer caches in RAM memory, UNIX also utilizes a RAM buffer to minimize unnecessary disk I/O. This buffer is commonly known as the Journal File System or JFS buffer. When Oracle data is retrieved from the Oracle database, the data block often travels through several layers of RAM caches (Figure 2-4).

Figure 4: Multiple UNIX RAM buffer caches

As we see in Figure 2-4, when Oracle makes a request to fetch a data block, the Oracle data buffer is first checked in ensure that the block is not already in the Oracle buffer. If it is not, the UNIX JFS buffer will then be checked for the data block. If the data block is not in the JFS buffer, the disk array buffer is then checked. Only when none of these three buffers contains the data block is a physical disk read incurred.

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The above is an excerpt from the "Oracle9i UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.

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