3Com SuperStack 3 WEBCACHE 1000, User Manual

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Viewing Performance Graphs 221
calculated as the ratio of bytes served by the Webcache to total
requested bytes. A high graph rating is desirable because it indicates
that the Webcache has reduced WAN bandwidth use. A low graph
rating indicates the opposite.
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Hit Rate
The hit rate reveals how effective the Webcache is at dealing with
HTTP requests sent by clients. A full hit occurs when the Webcache
serves the request without having to check with the origin server first.
A revalidated hit occurs when the Webcache serves the request, only
after checking with the original server that the content is current. A
cache miss happens when the Webcache is unable to serve a HTTP
request. The hit rate is determined by the total number of full hits and
revalidated hits against the total number of requests and is registered
as a percentage.
When the Webcache is first deployed, there will be a high number of
revalidated hits as the Webcache learns how to cache the content.
When the Webcache learns how to cache, you should expect a higher
percentage of full hits. However, some Web sites do not allow full
caching so, even though the number of revalidated hits is high, it does
not necessarily mean that there is a problem.
A high hit rate indicates a 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 good hit rate is 40-60%.
The hit rate that the Webcache achieves is largely dependant upon:
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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.
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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.
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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.
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Request Rate
The average number of client machine HTTP requests sent to the
Webcache per second. A high request rate — even up to the
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