Re: REDO LOG SPACE REQUESTS

"Daniel Roy" <danielroy10_at_hotmail.com> wrote in message news:1b061893.0304211808.43e3501a_at_posting.google.com...
> From Oracle:
>
> Number of times the active log file is full and Oracle must wait for
> disk space to be allocated for the redo log entries. Such space is
> created by performing a log switch.
>
> Log files that are small in relation to the size of the SGA or the
> commit rate of the work load can cause problems. When the log switch
> occurs, Oracle must ensure that all committed dirty buffers are
> written to disk before switching to a new log file. If you have a
> large SGA full of dirty buffers and small redo log files, a log switch
> must wait for DBWR to write dirty buffers to disk before continuing.
>
> Also examine the log file space and log file space switch wait events
> in V$SESSION_WAIT
>
> HTH
Hi Daniel,

Note the above is (thankfully) not correct. A log switch does not "wait for DBWR to write dirty buffers to disk before continuing", else the impact on performance could be nasty. I believe you're suggesting that it triggers a checkpoint, which is kinda true, but the log switch itself doesn't wait until the checkpoint completes, it simply changes the status of the redo log to "active" (from "current") and continues with the checkpoint in the background, finally changing the redo log status to "inactive" once complete. The redo log will not be overwritten until the redo log status is inactive.

Note also that even the "checkpoint" itself doesn't necessarily write buffers to disk if incremental checkpointing is being used.

Cheers

Richard Received on Tue Apr 22 2003 - 09:08:57 CDT