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88 CHAPTER 9: PERFORMANCE MONITORING
3 Select 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.
4 Click 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.