Re: latch-free SCN scheme (10.1.0.3)
- From: Martic Zoran <zoran_martic@xxxxxxxxx>
- To: gogala@xxxxxxxxxxxxx, oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 15 Feb 2005 14:43:45 -0800 (PST)
Mladen,
You do not need redo copy latch if the transaction is
small (not valid from 9i) or you have only 1 CPU
system and only one redo allocation latch.
In this case Oracle is turning this option off not
allowing this to happen at all.
Whenever you have multiple redo allocation latches no
way Oracle will not use redo copy latch(es).
The number of strands is telling you how many redo
allocation child latches you have.
But, that was the past. In 10g you can turned to not
use copy latces at all with:
_log_private_parallelism=true
then you will get zero-copy = TRUE.
That means no need to aquire copy latch at all :)
This is probably something that is going to be
standard setup for 11u :)
Regards,
Zoran
--- Mladen Gogala <gogala@xxxxxxxxxxxxx> wrote:
> In my alert.log I have the following text:
>
> Mon Feb 14 20:29:29 2005
> Starting ORACLE instance (normal)
> LICENSE_MAX_SESSION =3D 0
> LICENSE_SESSIONS_WARNING =3D 0
> Picked latch-free SCN scheme 2
> KCCDEBUG_LEVEL =3D 0
> Using LOG_ARCHIVE_DEST_10 parameter default value as
> USE_DB_RECOVERY_FILE_D=
> EST
> Autotune of undo retention is turned on.
> Dynamic strands is set to TRUE
> Running with 1 shared and 10 private strand(s).
> Zero-copy redo is FALSE
>
>
>
> What is "latch-free SCN scheme"? As far as I am
> aware, any transaction that=
> needed
> to increase SCN, needed to acquire latch that was
> protecting SCN. Is it sti=
> ll the=20
> case? V$LATCHNAME, of course, tells a different
> story:
>
> 1 select name,latch# from v$latchname
> 2 where name like '%SCN%'
> 3* order by 1
> SQL> /
>
> NAME LATCH#
> ------------------------- ----------
> batching SCNs 110
> change tracking consisten 155
> t SCN
>
> change tracking optimizat 154
> ion SCN
>
> flashback SCN barrier 159
> flashback hint SCN barrie 161
> r
>
>
> NAME LATCH#
> ------------------------- ----------
> lgwr LWN SCN 106
> mostly latch-free SCN 105
> ping redo on-disk SCN 108
> redo on-disk SCN 107
>
> 9 rows selected.
>
>
> What's the deal? If SCN acquisition is really latch
> free, ie does not requi=
> re=20
> previous latch acquisition, that would be a great
> performance enhancement.=20
> If that is so, is that true for RAC as well? What
> about local and global SC=
> N?
> My world is falling apart! Moreover, what is
> "zero-copy redo"?
> Once upon a time, there was a parameter which was
> determining the maximum
> size of redo entry that was written directly to log
> buffer. Everything
> greater then that was first formatted in the process
> buffer, space was
> then allocated in the redo buffer (by acquiring redo
> allocation latch, of
> course), and when redo copy latch was acquired, the
> user buffer was copied
> into the log buffer. Am I correct in my
> understanding that all processes=20
> need to acquire only allocation latch and that they
> will write directly
> into the log buffer? No copy latch necessary?
> Jonathan, please help!
> --=20
> Mladen Gogala
> Oracle DBA
>
>
> --
> //www.freelists.org/webpage/oracle-l
>
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