[fdo-trac] #431: Make SDF Cache size tunable

[FDO]

#431: Make SDF Cache size tunable
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   Reporter:  brentrobinson  |       Owner:  brentrobinson               
       Type:  enhancement    |      Status:  new                         
   Priority:  major          |   Milestone:  3.4.0                       
  Component:  FDO API        |     Version:  3.4.0                       
   Severity:  2              |    Keywords:  SDF insert performance cache
External_id:                 |  
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 The SDF provider has an in-memory cache of inserted and updated features.
 The cache limit is 500 features. When the cache size is exceeded, the
 features are flushed to the SDF file.

 For some data sets, a larger cache limit leads to better feature insertion
 performance, which can speed up the initial population of these SDF files.
 However, the amount of improvement varies greatly by data set.

 This ticket proposes a couple of modifications to make the SDF cache size
 adjustable:

  1. Add a new environment variable called SDF_MAXCACHESIZE. When set, the
 SDF provider sets the cache size to the number specified. If the variable
 is a non-positive number, not numeric, or not set then the cache size
 defaults to 500 features.

  2. Add a new SDF Provider connection parameter, called MaxCacheSize,
 which behaves the same way as the above environment variable. This allows
 the tuning of the cache size for individual SDF connections. If the
 environment variable is also set, the connection parameter takes
 precedence.

 The above provides the opportunity to try different cache sizes to arrive
 at the optimal sizes for particular data sets.

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Ticket URL: <http://trac.osgeo.org/fdo/ticket/431>
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