88CHAPTER 9: PERFORMANCE MONITORING

3Select Caching Performance Graphs from the available graph types. Select one of the three available time periods, Daily, Weekly, or Monthly, by clicking the appropriate radio button.

4Click OK.

The Caching Performance graphs show the following information, which is updated every 5 minutes:

■Bandwidth Saving (%)

The average percentage of bandwidth savings obtained through the use of the Webcache. This is calculated as the ratio of bytes served by the Webcache to total requested bytes.

■Hit and Miss Rate (%)

The percentage of client machine HTTP requests sent to the Webcache that have been served from the Webcache (a cache hit) and not served from the Webcache (a cache miss). A cache miss requires the content to be retrieved from the origin server over the Web.

A high hit rate indicates more efficient operation, as the Webcache is saving requests from being sent to the Web, which speeds up response time and reduces bandwidth use. A reasonable hit rate is

40-60%. The hit rate that the Webcache achieves is largely dependant upon:

■How frequently the same request is made. The hit rate will be low if there is no revisiting of sites. Caching only works well if the same request is made frequently. The smaller the range of requests made, the more effective the Webache will be and the higher the hit rate will be.

■Whether the content provider on the origin web site allows the content to be cached or not. Some content providers will prevent certain information from being cached.

■Whether the content is frequently changed on the origin web site. If so, the copy held on the Webcache must be discarded, and the new version retrieved. This is treated as a cache miss.

■Request Rate (sec-1)

The average number per second of client machine HTTP requests sent to the Webcache.