[Solr Wiki] Trivial Update of "SolrPerformanceProblems" by ShawnHeisey

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The "SolrPerformanceProblems" page has been changed by ShawnHeisey:
https://wiki.apache.org/solr/SolrPerformanceProblems?action=diff&rev1=46&rev2=47

Comment:
Slightly expanded info about the default zkClientTimeout.

  
  Regardless of the number of nodes or available resources, SolrCloud begins to have stability problems when the number of collections reaches the low hundreds.  With thousands of collections, any little problem or change to the cluster can cause a stability death spiral that may not recover for tens of minutes.  Try to keep the number of collections as low as possible.  These problems are due to how SolrCloud updates cluster state in zookeeper in response to cluster changes.  Work is underway to try and improve this situation.
  
- Because SolrCloud relies heavily on Zookeeper, it can be very unstable if you have underlying performance issues that result in operations taking longer than the [[SolrCloud#SolrCloud_Instance_ZooKeeper_Params|zkClientTimeout]].  Increasing that timeout can help, but addressing the underlying performance issues  will yield better results.  The default timeout (15 seconds) is quite long and should be more than enough for a well-tuned SolrCloud install.
+ Because SolrCloud relies heavily on Zookeeper, it can be very unstable if you have underlying performance issues that result in operations taking longer than the [[SolrCloud#SolrCloud_Instance_ZooKeeper_Params|zkClientTimeout]].  Increasing that timeout can help, but addressing the underlying performance issues  will yield better results.  The default timeout (15 seconds internally, and 30 seconds in most recent example configs) is quite long and should be more than enough for a well-tuned SolrCloud install.
  
  Zookeeper's design assumes that it has extremely fast read and write access to its database.  If the Zookeeper database is stored on the same disks that hold the Solr data, any performance problems with Solr will delay Zookeeper's access to its own database.  This can lead to a performance death spiral where each ZK timeout results in recovery operations which cause further timeouts.