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On Mon, 15 Nov 1999, Lars Marowsky-Bree wrote:
> Since you are doing load balancing at layer 4, I would assume that you got a
> lot of redundancy in the caches of the different real servers, right?
We could do, and there will be some - at some point I'll try and find out
how much, however we've found that if we use cache digest peering in an
M*N fashion it doesn't cause as much network traffic as you would expect.
One side effect this has is that there will be duplication of some content
across the cluster - but mainly stuff that is "popular now", which will
result in better loading on each machine to an extent.
For example - the front page to www.hotmail.com is extremely popular - by
having that in several caches inside the cluster (if it is :-) the
duplication helps to provide a secondary layer of load balancing. If there
was only one copy in the cluster all requests for that object would go to
just one machine causing problems for that machine potentially.
> How do you deal with the problem?
Overall our hit rate for the cluster (when calculated properly) is very
high for a root level cache - in the mid 30s and occasionally 40s now.
It would be interesting to see if we could get Level 7 functionality into
LVS to hash the requested URL, since this would boost productivity even
further, without having the problem I mention above. (it needs to be Level
7 rather than simply trying to, say, route on destination IP address since
we're a root level cache and clients can choose to use us or not.)
Most Level 7 switches are designed with an organisation who can control
the users to a large extent, and as such when we've tried them haven't
been suitable to our needs. LVS gives us alot we can't do with a commercial
L4 switch as well - such as the IP tunneling - which to my mind is one of
the best innovations.
Michael.
--
National & Local Web Cache Support R: G117
Manchester Computing T: 0161 275 7195
University of Manchester F: 0161 275 6040
Manchester UK M13 9PL M: Michael.Sparks@xxxxxxxxxxxxxxx
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