Object Hit/Byte Hit accounting with Multiple Instances

[Michael Hendrie <michael@xxxxxxxxxxxxx>]

Hello List,

I have server running 3 instances of squid-3.0.STABLE19 using a  
configuration similar to that documented at http://wiki.squid-cache.org/MultipleInstances 
.  Each instance has all other instance configured as siblings using  
the "proxy-only" directive to allow sharing of cache without  
duplicating objects.  This setup is working very well and has  
increased server performance by over 50%.

I'm now trying to get an accurate indication of byte savings I'm  
achieving with this configuration however I'm not sure that the  
calculations I'm using are giving the correct results.  Because each  
instance maintains a separate cache_dir this seems to be a little  
difficult to calculate.  When instance 1 records a request as a MISS  
it may in fact be a HIT (from an entire system point of view) if the  
object is retrieved from the cache of instance 2 or 3.

Using a combination of "squidclient mgr:counters" and SNMP, I grab  
counter values from each instance, tally and use the following formula  
to calculate the byte hit ratio:

(mgr:counters:client_http.hit_kbytes_out +  
snmp:cacheClientHTTPHitKb.sibling_addresses) /  
(mgr:counters:client_http.kbytes_out -  
snmp:cacheClientHTTPHitKb.sibling_addresses) * 100 = % cache byte hit  
ratio

Using this formula, I always seem to get inconsistencies between what  
squid reports and what my benchmarking tool reports (web-polygraph).   
In the few cases I've checked so far, squid is always reporting a 4-5%  
less byte hit than what web-polygraph reports.

Can anyone suggest a better formula to calculate byte hits from a  
multi-instance configuration?


Cheers

Mick