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RE: Redo Copy Latch

From: Diego Cutrone <>
Date: Tue, 5 Dec 2000 13:30:42 -0300
Message-Id: <>

Hi Kamal:

    Forgive my poor english.
    When Oracle needs to copy the redo entries in the log buffer, it needs to acquire the
"redo allocation latch" in order to establish the area within the log buffer where its going to copy this entry.
After that, if the size of the redo entry is smaller than the setting of "log_small_entry_max_size" it copies the entry directly to the log buffer without taking any "redo copy latch", just holding the redo allocation latch. (This applies only to Oracle 7, not Oracle 8).

    If the redo entry is bigger than the setting of "log_small_entry_max_size" (Oracle 7), or if you're under Oracle 8, the process realeses the redo allocation latch and it requests the "redo copy latch". Once it gets this latch, then the process copies the redo entry info into the log buffer (within the area it had reserved before).

    It's normal to see redo copy latch misses, because of the behavior of the LGWR. When LGWR writes entries to disk (redo log files) it has to take and hold the redo copies latch first and then the redo allocation latch as well, in order to establish what entries it has to copy to disk. While this procedure takes place, you may see "redo copy latch misses", thats because LGWR is holding all the redo copies latch and there are some other processes requesting them too.

Hope it helps

> Hi Gurus
> As per my understanding, when Oracle needs to copy the changes to the redo
> log buffer it needs to acquire the latch and there are two types of
> 'redo allocation' and 'redo copy'. And which type of latch need to be
> acquired is based on the size of change and the parameter
> 'log_small_entry_max_size' (in multiple CPU only). If the change is bigger
> than log_small_entry_max_size then the oracle will acquire 'redo
> allocation' latch else 'redo copy'. If I am missing something then please
> correct me.
> We are having 28 CPUs and set the log_small_entry_max_size to 0. So I
> not see anything in the redo copy latch gets but I am getting the figures
> redo copy latch as well. Can anyone explain me the reason why, please ?
> Thanks & regards,
> Kamal Sood
> --
> Please see the official ORACLE-L FAQ:
> --
> Author: Sood, Kamal
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Received on Tue Dec 05 2000 - 10:30:42 CST

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