[Cassandra Wiki] Update of "FAQ" by BrandonWilliams

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The "FAQ" page has been changed by BrandonWilliams.
http://wiki.apache.org/cassandra/FAQ?action=diff&rev1=105&rev2=106

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   * [[#unsubscribe|How do I unsubscribe from the email list?]]
   * [[#cleaning_compacted_tables|I compacted, so why did space used not decrease?]]
   * [[#mmap|Why does top report that Cassandra is using a lot more memory than the Java heap max?]]
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+  * [[#jna|I'm getting java.io.IOException: Cannot run program "ln" when trying to snapshot or update a keyspace]]
  <<Anchor(cant_listen_on_ip_any)>>
  
  == Why can't I make Cassandra listen on 0.0.0.0 (all my addresses)? ==
@@ -397, +397 @@

  
  The difference between normal I/O and mmap() is that in the mmap() case the memory is actually mapped to the process, thus affecting the virtual size as reported by top. The main argument for using mmap() instead of standard I/O is the fact that reading entails just touching memory - in the case of the memory being resident, you just read it - you don't even take a page fault (so no overhead in entering the kernel and doing a semi-context switch). This is covered in more detail [[http://www.varnish-cache.org/trac/wiki/ArchitectNotes|here]].
  
+ <<Anchor(jna)>>
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+ == I'm getting java.io.IOException: Cannot run program "ln" when trying to snapshot or update a keyspace ==
+ Updating a keyspace first takes a snapshot. This involves creating hardlinks to the existing SSTables, but Java has no native way to create hard links, so it must fork "ln". When forking, there must be as much memory free as the parent process, even though the child isn't going to use it all.  Because Java is a large process, this is problematic.  The solution is to install [[http://jna.java.net/|Java Native Access]] so it can create the hard links itself.
+