what stays in db buffer cache
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  1. #1
    Join Date
    Sep 2000
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    Chicago, IL
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    Question

    Hi all,
    when data is read from disk, is the whole extent read into the buffer cache or is it only the blocks ?

    I am guessing the answer is only the blocks, but just wanted to make sure.

    Thanks.

  2. #2
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    The buffer cache works with blocks, not extents.
    Jeff Hunter
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    http://marist89.blogspot.com/
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    "I pledge to stop eating sharks fin soup and will not do so under any circumstances."

  3. #3
    Join Date
    Apr 2001
    Posts
    14
    Hi,

    Just to add :

    In case of access based on Index it reads a block at a time.
    While in case of full table scan it reads multiple blocks as
    defined by db_file_multiblock_read_count para.

    ..deval

  4. #4
    Join Date
    Mar 2000
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    solution

    hi, 1st June 2001 12:24 hrs chennai

    BUFFERS acts as containers in MEMORY for the BLOCKS on the DISK to hold its values.

    Oracle manages the storage space in the datafiles of a database in units called DATA BLOCKS. A data block is the SMALLEST unit of I/O used by a database. In contrast, at the physical, operating system level, all data is stored in bytes. Each operating system has what is called a block size. Oracle REQUESTS DATA in MULTIPLES of Oracle DATA BLOCKS, not operating system blocks.

    You can set the data block size for each Oracle database when you create the database. This data block size should be a multiple of the operating system's BLOCK SIZE within the maximum limit to avoid unnecessary I/O. Oracle data blocks are the smallest units of storage that Oracle can USE or ALLOCATE.

    Oracle stores data in DATA BLOCKS . One data block corresponds to a specific number of bytes of physical database space on disk.

    The next level of logical database space is EXTENTS. An extent is a specific number of CONTIGUOUS data blocks allocated for storing a specific type of information

    There is no extent loading or reading in to memory.

    cheers

    padmam
    Attitude:Attack every problem with enthusiasam ...as if your survival depends upon it

  5. #5
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    Re: solution

    Hi, 1st June 2001 12:55 hrs chennai

    In the LRU list and DIRTY list maintained by Oracle in DB buffer cache is maintained in BUFFERS conventions .

    The DB_BLOCK_BUFFERS in init.ora is also number of buffers to that can be allocated at instance startup.

    Each buffer in the cache contains one Oracle block. The most
    recently used data from indexes, tables, views, etc. is stored in the buffer cache.

    The DB_BLOCK_SIZE determines the size of the oracle blocks

    Database buffers cache
    ---------------------------

    . DB_BLOCK_SIZE
    . DB_BLOCK_BUFFERS

    The total space occupied by database buffers would
    be equal to DB_BLOCK_SIZE * DB_BLOCK_BUFFERS.

    Sorry for putting so much stuff for your simple question the whole emphasis is to make very clear on Buffers point of info.

    cheers

    Padmam
    Attitude:Attack every problem with enthusiasam ...as if your survival depends upon it

  6. #6
    Join Date
    May 2001
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    Delhi
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    Lot of Thanks

    Hi Padmam,

    I must say that you are the best. Your style of answering the queries is great.

    --------------------------
    The Time has come ....

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