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Thread: Checkpoint and Log Switch

  1. #1
    Join Date
    Apr 2002
    Location
    France
    Posts
    118
    Hi All,

    1. I have seen that checkpoints occur when the redo log file is 90% full.
    2. Also seen that a checkpoint occurs when there is a log switch.

    Am I write if I say that :
    - A check point will occur when the log file is 90 % full and AGAIN when it is 100 % full ?
    - A check point will happen when manually forcing a log switch ?

    Thanks in advance.
    If there is no solution, it is because there is no problem - Shadok -

  2. #2
    Join Date
    Jun 2000
    Location
    Madrid, Spain
    Posts
    7,447
    90% of smallest redo log file size checkpoints is expected, it's incremental

    log switch also cause checkpoints but since incremental checkpoints are occurring very often this "interval" checkpoints are not as heavy as oracle 8

  3. #3
    Join Date
    May 2002
    Posts
    2,645
    From MetaLink:

    Purpose:
    ========

    The purpose of this article is to explain what 8i parameters effect
    checkpoints in Oracle 8i.


    Scope & Application:
    ====================

    This article is meant as an Oracle 8i tuning aid.


    8i Parameters that Influence Checkpoints:
    =========================================

    In Oracle8i, there are a couple of things that cause checkpointing to occur.
    They are:

    o FAST_START_IO_TARGET init.ora parameter.
    o LOG_CHECKPOINT_TIMEOUT init.ora parameter.
    o LOG_CHECKPOINT_INTERVAL init.ora parameter.
    o Size of the smallest log.

    Oracle continually calculates where the checkpoint position must be in
    order to satisfy these things, and writes dirty buffers in order of age
    to advance the checkpoint position such that all of these targets are
    met. When it advances the checkpoint, it does not advance it to the end
    of the log. Rather, it advances it far enough to satisfy all of the
    targets corresponding to the items above.


    FAST_START_IO_TARGET:

    Oracle continually calculates the number of IO operations that would be
    required to perform roll-forward should the database fail. If this number
    is greater than FAST_START_IO_TARGET, Oracle writes dirty buffers and
    advances the checkpoint to satisfy this target. FAST_START_IO_TARGET is
    only useable on 8i Entreprise Edition.


    LOG_CHECKPOINT_TIMEOUT:

    Assume this is set to 60. Oracle continually calculates the address of the
    redo record that was written 60 seconds ago. In order to satisfy this
    parameter, the checkpoint position must advance at least as far as this redo
    record. Should the checkpoint position point to a redo record older than
    this target position (written over 60 seconds ago), Oracle writes dirty
    buffers and advances the checkpoint until it points at a redo record written
    less than 60 seconds ago. Should the checkpoint position point to a redo
    record newer than this target position (written less than 60 seconds ago),
    Oracle does nothing to satisfy this target for it is already satisfied.


    LOG_CHECKPOINT_INTERVAL:

    Assume this is set to 1000. Oracle continually calculates the address of
    the redo record that was written 1000 records (OS blocks) ago. In order to
    satisfy this parameter, the checkpoint position must advance at least as far
    as this redo record. Should the checkpoint position point to a redo record
    written earlier than this target position (written over 1000 records before
    the record at the end of the log), Oracle writes dirty buffers and advances
    the checkpoint until it points at a redo record written less than 1000
    records ago. Should the checkpoint position point to a redo record newer
    than this target position (written less than 1000 records ago), Oracle does
    nothing to satisfy this target for it is already satisfied.


    SIZE OF THE SMALLEST REDO LOG:

    Oracle forces LOG_CHECKPOINT_INTERVAL to be no greater than 90% of the size
    of the smallest log file. This guarantees that when Oracle tries to do a log
    switch because it has filled a redo log file, the checkpoint position will
    have advanced into this current log. Should you only have two logs (the
    worst case), Oracle does not have to stall while the checkpoint advances out
    of the soon to be reused log.


    So, when Oracle does a checkpoint due to a log switch, it does not really
    have to do anything. The checkpoint position is already advanced out of the
    soon to be reused log. The log switch checkpoint does not actually do
    anything right away. It is a "zero priority" checkpoint: it does not cause
    any writes to happen on its own, but is marked as complete only when
    checkpointing for other reasons causes the checkpoint position to advance
    beyond the rba that marked the end of the log when the checkpoint was initiated.


    Example:
    --------

    Assume you have two logs, A and B. When B fills, Oracle initiates a zero
    priority checkpoint to advance the checkpoint position to the end of log B.
    However, because the checkpoint position is no more than 90% of the size of
    the smallest log behind the end of the log, the checkpoint position is roughly
    10% of the way into log B when this checkpoint is initiated. There is no
    reason to do any writes, as log A can be reused immediately. So Oracle does
    nothing and lets the checkpoint position continue to advance based upon other
    factors. When the other factors cause the checkpoint position to advance
    beyond the end of log B, the zero priority checkpoint previously initiated is
    declared complete and you see the checkpoint complete message.

    Note: If you manually switch log files, the checkpoint position may not
    have advanced out of the log to be reused. In this case, Oracle
    initiates a real checkpoint and advances the checkpoint position
    up to the rba that marked the end of the log when the checkpoint
    was initiated.

  4. #4
    Join Date
    Apr 2002
    Location
    France
    Posts
    118
    Thanks to both of you.
    If there is no solution, it is because there is no problem - Shadok -

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