DB buffer Cache/Page-Swap
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  1. #1
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    DB buffer Cache/Page-Swap

    Hi.. I have some notes on DB Buffer Cache Parameter from a while ago and was wondering about one thing..

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    DOC>Notes on DB Block Buffers (Buffer Cache Hit Ratios)
    DOC>In turn, the setting of this parameter has a large effect on the buffer cache hit ratio, which you generally want to be above 90%. The hit ratio can be dynamically determined based on the following query: If the hit ratio is below 90%, then it would be advisable to increase DB_Block_Buffers until you obtain a hit ratio above 90%. The implications of the DB_Block_Buffers being set too low are that the least recently used data will be flushed from memory. If another query needs this data, back out to disk we go. I think we can see this will cause I/O and CPU resources to be used.If the value is set too high, swapping will began to occur on the OS and the system may come to a halt.

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    In the last sentence, where it says, value high or low, I believe it should be low... What are the effects, if we have too much SGA allocation? My parameters are all looking good, but system is Paging and swapping at 40%. Is it good or bad or normal..

    Env: Oracle 817 on AIX 433
    DBsize: 150GB and SGA about 375MB.
    Another 4 instances at 50GB with 500MB SGA.
    Paging and swapping: set to 4.5GB (across 4 disks)
    Total Filesystems space: 240G, RAID 1
    System is OLTP and have batch runs before business hours.

    Thanks, ST2000

  2. #2
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    How much total memory do you have on your AIX server?
    David Knight
    OCP DBA 8i, 9i, 10g

  3. #3
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    Re: DB buffer Cache/Page-Swap

    Originally posted by st2000
    I think we can see this will cause I/O and CPU resources to be used.If the value is set too high, swapping will began to occur on the OS and the system may come to a halt.

    -------

    In the last sentence, where it says, value high or low, I believe it should be low...

    No, the sentance is correct. If your db_block_buffers is too large, it will consume lots of real memory thereby forcing you to swap.

    What are the effects, if we have too much SGA allocation?

    You will swap processes out to disk. Since disk is much slower than RAM, your performance will suffer.

    My parameters are all looking good, but system is Paging and swapping at 40%. Is it good or bad or normal..
    I try to avoid any swapping.
    Jeff Hunter
    marist89@yahoo.com
    http://marist89.blogspot.com/
    Get Firefox!
    "I pledge to stop eating sharks fin soup and will not do so under any circumstances."

  4. #4
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    Memory on the box is 4GB, 2 Processes.

    Thanks for the info Jeff.. What are the steps to take, to avoid paging and swapping.. I mean, should I increase the components of SGA and what should be the size of Page/swap on a machine like this..

    Thanks, ST2000
    Last edited by st2000; 12-03-2002 at 01:43 PM.

  5. #5
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    I have checked the swapping/paging and since a week it is consistent around 30%. Please advise..

    Thanks, ST2000

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