Chapter 2, Views
System Buffering View
Select View, System, Buffering.
Buffer Hit Ratios View
- Function:
- Description:
- This view displays a vertical bar graph showing the read and write cache hit ratios.
- The operating system keeps a store (cache) of buffers to keep the most recently accessed view data in memory. This enables the system to retrieve required data from the cache rather than having to read the data from disk.
- The buffer cache also benefits write requests when the amount of data to be written is less than a complete system block. In this case, the system must first read the block from disk before applying the write request to that block. However, if the block is already in the buffer cache, the new data can be updated without a disk read.
- The ratio of block accesses, found in the cache, to total accesses is called the Hit Ratio. The higher the ratio the better. Read Hit Ratios above 90% and Write Hit Ratios of approximately 70% are typical.
- When running application programs, you generally want the buffer cache to be as large as possible, without causing memory shortages, which could cause a paging bottleneck (which is worse still). A good general guide is to make the buffer cache 10% of system memory and to increase this to 15% if you have plenty of memory available.
- As the cache hit ratios are subject to dramatic changes, this view defaults to displaying the average instance. Good average cache hit values are important for overall system performance. If the ratios appear low, it may be caused by the system buffer cache being too small. In this case, check the system buffer cache size via the System Memory View.