SGA calculation
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Thread: SGA calculation

  1. #1
    Join Date
    Oct 2000
    Posts
    28

    Wink

    Hello,
    SQL> select * from v$sga;

    NAME VALUE
    -------------------- ---------
    Fixed Size 65484
    Variable Size 19521536
    Database Buffers 16777216
    Redo Buffers 73728


    In above queries database buffer size comes from db_block_buffers = 8192 # INITIAL parameter of init.ora file. But other parameters like fixed size ,var. size , redo buffers are coming from where ?

    How to calculate the memory occupied by an instance ?

    Sunil

  2. #2
    Join Date
    Oct 2000
    Location
    Saskatoon, SK, Canada
    Posts
    3,925


    Sample from svrmgrl SHOW SGA:

    Total System Global Area 23460696 bytes
    Fixed Size 72536 bytes
    Variable Size 22900736 bytes
    Database Buffers 409600 bytes
    Redo Buffers 77824 bytes

    Total System Global Area
    - Total in bytes of all the sub-divisions that makes up the SGA.

    Fixed Size
    - Contains general information about the state of the database and the instance, which the background processes need to access.
    - No user data is stored here.
    - This area is usually less than 100k in size.

    Variable Size
    - This section is influenced by the following init.ora parameters
    shared_pool_size
    large_pool_size
    java_pool_size
    - See 'Approximating Size of the SGA' section of this article for version specific information.

    Database Buffers
    - Holds copies of data blocks read from datafiles.
    size = db_block_buffers * block size

    Redo Buffers
    - A circular buffer in the SGA that holds information about changes made to the database.
    - Enforced mininum is set to 4 times the maximum database block size for the host operating system.
    To approximate size of the SGA (Shared Global Area), use the following formula:

    ((db_block_buffers * block size) +
    (shared_pool_size + large_pool_size + java_pool_size + log_buffers) + 1MB




    Sam



    [Edited by sambavan on 10-30-2001 at 04:33 PM]
    Thanx
    Sam



    Life is a journey, not a destination!


  3. #3
    Join Date
    Jun 2001
    Location
    Helsinki. Finland
    Posts
    3,938
    To approximate size of the SGA (Shared Global Area), use the following formula:

    ((db_block_buffers * block size) +
    (shared_pool_size + large_pool_size + java_pool_size + log_buffers) + 1MB

    To Sam's good output, I would like to add that in case you use a multi-instanced DB, you have to add (in the formula above) the Instance Lock Database.

    You can calculate your locks by changing the init.ora parameter GC_FILES_TO_LOCKS and counting the difference. Results are platform dependent but I would say that 64 bytes for a PCM lock is a good estimate.


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