optimization : check if storage directory exists, if rrd file is writable and use interval lenght by function 
optimization : check if storage directory exists, if rrd file is writable and use interval lenght by function 

git-svn-id: 033098717d16b8835dc835613dd6a3f517fdbf03@3128 6bcd3966-0018-0410-8128-fd23d134de7e

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