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SOS Oracle Detail DBWR

The Oracle Detail DBWR Screen

The Oracle Detail DBWR screen provides statistics about the DBWR process; the process that writes the modified buffers into a database.
To access the Oracle Detail DBWR screen from any SOS display screen:
  • Type s from the SOS Enter command: prompt to view the Screen Selection Menu.
  • From the Screen Selection Menu, enter B (upper case). An example of the screen is shown in Figure 41.1.






  • Figure 41.1 SOS Oracle Detail DBWR screen

    Oracle Detail DBWR Display Items

    DBWR

    The DBWR section displays information about the mirror write consistency (MWC) DBWR.
    Table 41.1 SOS Oracle Detail DBWR data items: DBWR
    Data Item
    Description
    Chkpnt Buf Wrtn
    The number of buffers that were written for checkpoints.
    Trans Table Wrts
    The number of transaction table writes.
    Undo Block Writes
    The number of transaction table blocks written by DBWR. This value is an indication of how many "hot" buffers were written, leading to write complete waits.
    Rev bng-wrtn Buf
    The number of times that DBWR tried to save a buffer for writing and found that it was already in the write batch.
    This statistic is a measure of the amount of "unnecessary" work that DBWR had to do in trying to fill the batch. This can occur because many sources contribute to a write batch. If the same buffer from different sources is considered for adding to the write batch, then all but the first attempt will be unnecessary since the buffer is already marked as being written.
    Make Free Reqs
    The number of requests to make more buffers free in the LRU section of the buffer cache.
    Free Bufs Found
    The number of free buffers found that DBWR found to be free when requested to make free buffers.
    The average number of free buffers is:
    (DBWR free buffers found / DBWR make free requests)
    LRU Scans
    The number of scans through LRU chain for more buffers to write.
    Sum Scans Depth
    Can be divided by LRU scans to determine the average length of the scans through the buffer cache. It is not the number of buffers scanned.
    If the write batch is filled and a write takes place to disk, the scan depth halts.
    Bufs Scanned
    The number of buffers in the LRU section of the buffer cache scanned by DBWR when it searches for dirty buffers to write to disk. It does not halt as summed scans depth.
    The average number of buffers being scanned is calculated:
    (DBWR buffers scanned / DBWR LRU scans)
    Chkpnts
    The number of times DBWR was signaled to perform a checkpoint by LGWR.
    Forced Writes
    The number of blocks forced written.

    Background Checkpoints

    The Background Checkpoints section data items are described in the next table.
    Table 41.2 SOS Oracle Detail DBWR data items: Background Checkpoints
    Data Item
    Description
    Started
    The number of background checkpoints started.
    Completed
    The number of background checkpoints completed.
    Performance Tip
    The background checkpoints started and completed values should differ by 1. If the difference is greater than 1, DBWR is falling behind, and the size of the log files should be increased, or the buffer cache is too small.

    Inspected

    The Inspected section data items are described in Table 41.3.
    Table 41.3 SOS Oracle Detail DBWR data items: Inspected
    Data Item
    Description
    Dirty Buffers
    The number of dirty buffers found in the cache.
    If the value is large or continues to increase, DBWR is not keeping up with the workload.
    Free Buffers
    The number of free buffers found in the cache.

    Misc

    The Misc section data items are described in the next table.
    Table 41.4 SOS Oracle Detail DBWR data items: Misc
    Data Item
    Description
    Phys Writes
    The number of writes to disk, to both data files and log files made by DBWR and LGWR. This statistic is incremented by 1 regardless if it was a single-block write or a multiple-block write.
    Summed Dirty Qlen
    The sum of the dirty LRU queue length after every write request. Divide by write requests to get the average queue length after write completion.

    Requests

    The Requests section data item is described in the following table.
    Table 41.5 SOS Oracle Detail DBWR data item: Requests
    Data Item
    Description
    Free buffers
    The number of free buffer requests. A free buffer request happens when data is inserted into a database, every time a new block is required. When data is updated, free buffers are requested to contain rollback information.

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