However, increasing the cache size to 334MB would potentially decrease reads by a factor of .93 or 7%. If an additional 30MB memory is available on the host system and theSGA_MAX_SIZE setting allows the increment, it would be advisable to increase the default buffer cache pool size to 334MB.
但是,增加缓存到334M将可能减少7%左右的物理读,如果在当前系统有额外30M的内存的可以被使用和通过设置sga_max_size的大小到增加,增加默认的调整缓存到334M是可取的.
Estd Phys Estd Phys
Cache Size (MB) Buffers Read Factor Reads
---------------- ------------ ----------- ------------
30 3,802 18.70 192,317,943 10% of Current Size
60 7,604 12.83 131,949,536
91 11,406 7.38 75,865,861
121 15,208 4.97 51,111,658
152 19,010 3.64 37,460,786
182 22,812 2.50 25,668,196
212 26,614 1.74 17,850,847
243 30,416 1.33 13,720,149
273 34,218 1.13 11,583,180
304 38,020 1.00 10,282,475 Current Size
334 41,822 .93 9,515,878
364 45,624 .87 8,909,026
395 49,426 .83 8,495,039
424 53,228 .79 8,116,496
456 57,030 .76 7,824,764
486 60,832 .74 7,563,180
517 64,634 .71 7,311,729
547 68,436 .69 7,104,280
577 72,238 .67 6,895,122
608 76,040 .66 6,739,731 200% of Current Size
This view assists in cache sizing by providing information that predicts the number of physical reads for each potential cache size. The data also includes a physical read factor, which is a factor by which the current number of physical reads is estimated to change if the buffer cache is resized to a given value.
这个视图帮助提供缓存大小的信息,估计每个可能的缓存大小的物理读.数据也包括物理读的因素,如果调整buffer cache的给定值,当前的物理读可能会发生改变.
The relationship between successfully finding a block in the cache and the size of the cache is not always a smooth distribution. When sizing the buffer pool, avoid the use of additional buffers that contribute little or nothing to the cache hit ratio. In the example illustrated in Figure 7-1, only narrow bands of increments to the cache size may be worthy of consideration.
成功的在缓存中找到一个块和缓存大小不总是正常的分配之间的关系.在设置缓存池的大小,避免增加缓存而只有提升了很少的缓存命中率,在7-1的事例说明,增加了缓存的大小只有很小幅度就值得被重新考虑.
Figure 7-1 Physical I/O and Buffer Cache Size
Examining Figure 7-1 leads to the following observations:
The benefit from increasing buffers from point A to point B is considerably higher than from point B to point C.
The decrease in the physical I/O between points A and B and points B and C is not smooth, as indicated by the dotted line in the graph.
查看7-1可以得到如下的观察结果:
从中可以得到从A到B增加内存比从B到C增加高很多.
减少物理读,在AB和BC是不平滑的,如图中的虚线.
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