Mladen, I finally found the doc I was searching for (my knowledge base is very messy): http://integrid.info/memory_latching.pdf This guy explains also latchless SCN in some details. Great idea, like every transaction own 255 SCN numbers, then no need to take the latch :) The similar mechanism is for zero copy latch where sessions have preallocated log buffer space. That means no need for redo copy latch at all. Very cool document. 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 > __________________________________ Do you Yahoo!? Yahoo! Mail - Easier than ever with enhanced search. Learn more. http://info.mail.yahoo.com/mail_250 -- //www.freelists.org/webpage/oracle-l