tailieunhanh - Sizing the Buffer Cache

After completing this lesson, you should be able to do the following: Employ the buffer cache sizing advisor Describe how the buffer cache is used by different Oracle processes Create and manage different buffer caches Monitor the use of the buffer caches Identify and resolve buffer cache performance problems | Sizing the Buffer Cache Objectives After completing this lesson, you should be able to do the following: Employ the buffer cache sizing advisor Describe how the buffer cache is used by different Oracle processes Create and manage different buffer caches Monitor the use of the buffer caches Identify and resolve buffer cache performance problems Overview Data files SGA DB buffer cache Checkpoint Queue . . . . LRU lists DBWn Server DB_BLOCK_SIZE DB_CACHE_SIZE DB_KEEP_CACHE_SIZE DB_RECYCLE_CACHE_SIZE DB_BLOCK_CHECKSUM . . . . Buffer Cache Characteristics The buffer cache holds copies of the data blocks from the data files. The buffer cache is a part of the SGA; so all users can share these blocks. The server processes read data from the data files into the buffer cache. To improve performance, the server process sometimes reads multiple blocks in a single read. The DBWn process writes data from the buffer cache into the data files. To improve performance, DBWn writes multiple blocks in a single write. At any given time, the buffer cache may hold multiple copies of a single database block. Only one current copy of the block exists, but to satisfy queries server processes may need to construct read-consistent copies from past image information(a CR block). The Least Recently Used list monitors the usage of buffers. The buffers are sorted in accordance with the number of times that they are used. Thus, buffers that are frequently used will be found at the most recently used end, whereas those buffers that are least used are found at the least recently used end. Incoming blocks are copied to a buffer from the least recently used end, which is then moved to the middle of the list. From here the buffer will work its way up or down the list depending on usage. Buffer Cache Characteristics (continued) Buffers in the buffer cache can be in one of four states: Pinned: Meaning that multiple sessions are kept from writing to the same block at the same time. Other sessions wait to | Sizing the Buffer Cache Objectives After completing this lesson, you should be able to do the following: Employ the buffer cache sizing advisor Describe how the buffer cache is used by different Oracle processes Create and manage different buffer caches Monitor the use of the buffer caches Identify and resolve buffer cache performance problems Overview Data files SGA DB buffer cache Checkpoint Queue . . . . LRU lists DBWn Server DB_BLOCK_SIZE DB_CACHE_SIZE DB_KEEP_CACHE_SIZE DB_RECYCLE_CACHE_SIZE DB_BLOCK_CHECKSUM . . . . Buffer Cache Characteristics The buffer cache holds copies of the data blocks from the data files. The buffer cache is a part of the SGA; so all users can share these blocks. The server processes read data from the data files into the buffer cache. To improve performance, the server process sometimes reads multiple blocks in a single read. The DBWn process writes data from the buffer cache into the data files. To improve performance, DBWn writes multiple blocks in a