[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=53&rev2=54

Comment:
Indicate there is no formula for minimum memory size.  Link "Sizing Hardware" blog post.

  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 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.
+ 
+ /!\ It's very important to note here that there is no quick formula available for deciding the minimum amount of memory required for good performance.  Setting up a full system may be the only way you can know.  Sometimes a small-scale prototype can reveal useful information.  See [[https://lucidworks.com/blog/sizing-hardware-in-the-abstract-why-we-dont-have-a-definitive-answer/|this article]] for a discussion on this topic.
  
  ==== Special note regarding the optimize operation ====