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缓冲区高速缓存的命中率 的定义是什么?
总结如下:
命中率=1-(physical reads/(db blockgets+consistent gets)
上面的定义是正确的!其中:
consistent gets :
This statistics reports the number of blocks requested in consistent mode. Blocks are requested in consistent mode for consistent read operations. When a block is requested in consistent mode, if any changes have been committed or made to that block since the requesting statement (or transaction) began, then they must be rolled back for the purposes of the read, to give a consistent view of the data at that time.
db block gets :
This statistics counts the number of blocks requested in current mode. Blocks are requested in current mode in order to be changed.
physical reads :
This is the number of database blocks that the database instance has requested the operating system to read from disk. This is typically greater than the actual number of disk read operations performed because of multiblock reads and because some read requests may be satisfied from the operating system buffer cache
但命中率取值:
Calculate buffer cache hit ratio in the database. Make sure it is more than 80 for an oltp environment and 99 is the best value. |
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