[Cassandra Wiki] Update of "FAQ" by EricEvans

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The "FAQ" page has been changed by EricEvans.
The comment on this change is: people keep asking....
http://wiki.apache.org/cassandra/FAQ?action=diff&rev1=31&rev2=32

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   * [[#what_kind_of_hardware_should_i_use|What kind of hardware should I run Cassandra on?]]
   * [[#architecture|What are SSTables and Memtables?]]
   * [[#working_with_timeuuid_in_java|Why is it so hard to work with TimeUUIDType in Java?]]
+  * [[#i_deleted_what_gives|I delete data from Cassandra, but disk usage stays the same. What gives?]]
  
  <<Anchor(cant_listen_on_ip_any)>>
  == Why can't I make Cassandra listen on 0.0.0.0 (all my addresses)? ==
@@ -152, +153 @@

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+ <<Anchor(i_deleted_what_gives)>>
+ == I delete data from Cassandra, but disk usage stays the same. What gives? ==
+ Data you write to Cassandra gets persisted to SSTables. Since SSTables are immutable, the data can't actually be removed when you perform a delete, instead, a marker (also called a "tombstone") is written to indicate the value's new status. Never fear though, on the first compaction that occurs after ''GCGraceSeconds'' (hint: storage-conf.xml) have expired, the data will be expunged completely and the corresponding disk space recovered.
+