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  1. #1
    Join Date
    Oct 2002
    Breda, The Netherlands

    How to determine minimal extent size ?


    I have 40 Kilobytes as an extent size in my tablespaces. Now I read the following text on this page.
    Having 1,000 extents for an object by itself does not pose any performance problems, so long as the extents are sized as a multiple of (DB_FILE_MULTIBLOCK_READ_COUNT * DB_BLOCK_SIZE). The aforementioned formula is relevant for full-table scans or index fast full scans and not for single-block index scans. If the extent sizes adhere to the above formula, it ensures that Oracle will issue the same number of read system calls regardless of whether the object has 1 extent or 1,000 extents. If the extents are not aligned with the aforementioned size, additional read system calls can cause unnecessary overhead on the I/O sub-system. In the bigger scheme of things, assuming worst-case scenario, go ahead and add one additional read system call per extent per heavily hit table/index in your database. If you have many hundreds of objects with 1000s of misaligned extents, it will create overhead on the I/O sub-system.
    In my 8i database:
    db_file_multiblock_read_count = 128
    db_block_size = 8 Kilobytes

    Does this mean that the minimal extent size must be 128 * 8 = 1024KB?

    Help is very appreciated.

    Last edited by efrijters; 09-25-2003 at 07:38 AM.
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