[Cassandra Wiki] Update of "ArchitectureOverview" by StuHood

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The "ArchitectureOverview" page has been changed by StuHood.
The comment on this change is: Without an explanation for where the numbers came from, this blurb is useless..
http://wiki.apache.org/cassandra/ArchitectureOverview?action=diff&rev1=8&rev2=9

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  You get consistency if R + W > N, where R is the number of records to read, W is the number of records to write, and N is the replication factor.  A ConsistencyLevel of ONE means R or W is 1.  A ConsistencyLevel of QUORUM means R or W is ceiling((N+1)/2).  A ConsistencyLevel of ALL means R or W is N.  So if you want to write with a ConsistencyLevel of ONE and then get the same data when you read, you need to read with ConsistencyLevel ALL.
  
- Cassandra vs MySQL with 50GB of data
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- || MySQL       || Cassandra    ||
- ||~300ms write || ~0.12ms write||
- ||~350ms read  || ~15ms read   ||
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