[Solr Wiki] 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=17&rev2=18

Comment:
cleaned up 'OS disk cache' section a little bit.

  
  For index updates, Solr relies on fast bulk reads and writes. For search, fast random reads are essential. One way to satisfy these requirements is to ensure that a large [[http://en.wikipedia.org/wiki/Page_cache|disk cache]] is available, another is it use Solid State Drives (see the SSD section for details). See [[http://blog.thetaphi.de/2012/07/use-lucenes-mmapdirectory-on-64bit.html|Uwe's blog entry]] for a lot of good Lucene/Solr specific information.
  
- In a nutshell, you want to have enough memory available in the OS disk cache so that the important parts of your index, or ideally your entire index, will fit into the cache.  Let's say that you have a Solr index size of 8GB.  If your OS, Solr's Java heap, and all other running programs require 4GB of memory, then an ideal memory size for that server is at least 12GB.  The exact minimum requirements are highly variable and depend on things like your schema, your index contents, and your queries.
+ In a nutshell, you want to have enough memory available in the OS disk cache so that the important parts of your index, or ideally your entire index, will fit into the cache.  Let's say that you have a Solr index size of 8GB.  If your OS, Solr's Java heap, and all other running programs require 4GB of memory, then an '''ideal''' memory size for that server is at least 12GB.  You ''might'' be able to make it work with 8GB total memory (leaving 4GB for disk cache), but that also might NOT be enough.
  
- The exact amount of disk cache RAM required is difficult to judge, but if your index has a lot of stored fields, those requirements would be on the smaller end of the scale.  If you have very little stored data, you would want to be on the higher end of the scale.  The size of stored data doesn't affect search speed very much, though it might affect how long it takes to retrieve the search results once the required documents have been determined.
+ The exact minimum requirements are highly variable and depend on things like your schema, your index contents, and your queries.  If your index has a lot of stored fields, those requirements would be on the smaller end of the scale.  If you have very little stored data, you would want to be on the higher end of the scale.  The size of stored data doesn't affect search speed very much, though it might affect how long it takes to retrieve the search results once the required documents have been determined.
  
  === Java Heap ===