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This article mainly explains "how to understand buffer cache in Oracle". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let's let Xiaobian take you to learn "how to understand buffer cache in Oracle"!

1. Buffer cache

Database buffers are part of Oracle's memory structure, SGA.

2. Buffer cache significance

(1) Cache dbf files.

Buffer exists in memory, dbf exists on disk. When reading data from disk, dbf data will first be sent to buffer cache to achieve the role of cache.

(2) Structural cr block

In a session, users delete a piece of data but do not commit it. The data before the change is placed in the undo segment. When another session reads the data of the same block and finds that the data in the block has been deleted, a new block will be generated in the buffer cache. This new block is generated by the old block, and the deleted data will be supplemented by undo. This new block is called cr block.

So when one session deletes a piece of data without committing it, the other session sees the data before it was deleted.

3. Database Write Process (DBWn)

When a block in the buffer cache is modified, the modified block is called a dirty block. DBWn is responsible for writing dirty blocks from buffer cache to dbf files.

DBWn is not triggered every time a data block is changed. Even when triggered, not all blocks are written into dbf files, but some infrequently used blocks are written in. It has certain trigger conditions.

(1) Triggers every 3 seconds

(2) When the data is to be written into the buffer cache, it is found that there is not enough space.

(3) When performing checkpoint processes

4. Important parameters configuration of Buffer cache

(1) Query the size of each component of sga

SQL> select component,current_size/1024/1024 current_M,min_size/1024/1024 min_M from v$sga_dynamic_components;

(3) Modify buffer cache size

SQL> alter system set db_cache_size=20M scope=both;

In general, for DB_CACHE_SIZE settings, the recommended configuration is:

1/2~2/3 of SGA_MAX_SIZE

At this point, I believe that everyone has a deeper understanding of "how to understand buffer cache in Oracle," so let's actually operate it! Here is the website, more related content can enter the relevant channels for inquiry, pay attention to us, continue to learn!

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