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  1. #1
    Join Date
    Apr 2004

    Unanswered: light scans and buffer caching


    I am working on XPS 8.32 on HP-UX. we are doing a capacity planning and performance benchmarking for one of the project. when i do a onstat -g scn i find that the light scans are happening. when i do "onstat -p" the buffer reads are much less compared to diskeads(Caching is 0). Also the disk utilization is 100%. I feel that there is no caching happening in the lightscan buffers and all the requests are directed to disk. Will this behaviour not effect the overall performance.


  2. #2
    Join Date
    Mar 2004

    Yes this of course couold have an impact on your performance. The rough idea about light scans is the following:
    When XPS expects for example that the resultset from a query is larger as 75% of the buffercache it wil use light scans. Light scan are performed directly from disk to a big buffer in the virtual portion of the engine. The buffercahce isn't touched. Caching would be rather ridiculous because when restarting the query all the data have to be read again. Also putting data for a single read in the cache is rather stupid because:
    1. Something else (which might be used often) need to be kicked out
    2. Putting data in the buffercahce is rather complex, we need to shift LRU
    pointers etcetera.

    XPS will determine to use the vcache based on the parameters set and the way the query is built-up and run.

    Hope this helps,

    Rob Prop

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