[Cassandra Wiki] Update of "FAQ" by thepaul

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The "FAQ" page has been changed by thepaul:
http://wiki.apache.org/cassandra/FAQ?action=diff&rev1=123&rev2=124

Comment:
document what i've found about ubuntu/ec2/jna/memlock "task blocked for more than 120 seconds" problems

   * [[#seed_spof|Does single seed mean single point of failure?]]
   * [[#jconsole_array_arg|Why can't I call jmx method X on jconsole? (ex. getNaturalEndpoints)]]
   * [[#max_key_size|What's the maximum key size permitted?]]
+  * [[#ubuntu_ec2_hangs|I'm using Ubuntu on EC2 with JNA, and holy crap weird things keep hanging and stalling and printing scary tracebacks in dmesg!]]
  
  <<Anchor(cant_listen_on_ip_any)>>
  
@@ -476, +477 @@

  
  Routing is O(N) of the key size and querying and updating are O(N log N). In practice these factors are usually dwarfed by other overhead, but some users with very large "natural" keys use their hashes instead to cut down the size.
  
+ <<Anchor(ubuntu_ec2_hangs)>>
+ 
+ == I'm using Ubuntu on EC2 with JNA, and holy crap weird things keep hanging and stalling and blocking and printing scary tracebacks in dmesg! ==
+ 
+ We have come across several different, but similar, sets of symptoms that might match what you're seeing. They might all have the same root cause; it's not clear. One common piece is messages like this in dmesg:
+ 
+ {{{
+ INFO: task (some_taskname):(some_pid) blocked for more than 120 seconds.
+ "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
+ }}}
+ 
+ It does not seem that anyone has had the time to track this down to the real root cause, but it does seem that upgrading the linux-image-virtual package and rebooting your instances fixes it. There is likely some bug in several of the virtual/xen kernel builds distributed by Ubuntu which is fixed in later versions. Versions of linux-image-*-virtual which are known not to have this problem include:
+ 
+  * linux-image-2.6.38-10-virtual (2.6.38-10.46) (Ubuntu 11.04/Natty Narwhal)
+  * linux-image-2.6.35-24-virtual (2.6.35-24.42) (Ubuntu 10.10/Maverick Meerkat)
+ 
+ Uninstalling libjna-java or recompiling Cassandra with CLibrary.tryMlockall()'s mlockall() call commented out also make at least some sorts of this problem go away, but that's a lot less desirable of a fix.
+ 
+ If you have more information on the problem and better ways to avoid it, please do update this space.
+