latch-free SCN scheme (10.1.0.3)
- From: Mladen Gogala <gogala@xxxxxxxxxxxxx>
- To: oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Tue, 15 Feb 2005 01:52:29 +0000
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
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