[Cassandra Wiki] Update of "CassandraHardware" by JonathanEllis

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The "CassandraHardware" page has been changed by JonathanEllis.
http://wiki.apache.org/cassandra/CassandraHardware?action=diff&rev1=15&rev2=16

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  === Memory ===
- The most recently written data resides in memory tables (aka [[MemtableThresholds|memtables]]), but older data that has been flushed to disk can be kept in the OS's file-system cache. In other words, ''the more memory, the better'', with 1GB being the minimum we typically recommended in a virtualized environment.  Obviously there is no benefit to having more RAM than your hot data set, but with dedicated hardware there is no reason to use less than 8GB or 16GB, and you often see clusters with 32 GB or more per node.
+ The most recently written data resides in memory tables (aka [[MemtableThresholds|memtables]]), but older data that has been flushed to disk can be kept in the OS's file-system cache. In other words, ''the more memory, the better'', with 4GB being the minimum we typically recommended in a virtualized environment (e.g., EC2 Large instances).  Obviously there is no benefit to having more RAM than your hot data set, but with dedicated hardware there is no reason to use less than 8GB or 16GB, and you often see clusters with 32 GB or more per node.
  
  RAM can also be useful for the key cache (introduced in 0.5) and row cache (in 0.6).