Re: latch-free SCN scheme (10.1.0.3)

  • From: Connor McDonald <hamcdc@xxxxxxxxxxx>
  • To: oracle-l <oracle-l@xxxxxxxxxxxxx>
  • Date: Tue, 15 Feb 2005 23:58:20 +0000 (GMT)

There appears to be two enhancements..

In 9i, they introduced "log_parallelism" which is more than a single redo 
allocation latch.  In
10g, this parameter becomes hidden because the amount of parallelism is 
auto-decided/tuned by
Oracle.  So there's 3 parameters:

_log_parallelism
_log_parallelism_dynamic
_log_parallelism_max

Also in 10g, there a new thing called private redo.  All pretty much conjecture 
on my part here
but it looks like some of the log buffer gets created in such a way that 
sessions can directly
copy into it without requiring latches.  At a guess, "10 private strands" means 
up to 10 sessions
can get benefit from this mechanism at any given instance...from x$ksppi

_log_private_mul             - Private strand multiplier for log space 
preallocation
_log_private_parallelism     - Number of private log buffer strands for 
zero-copy redo
_log_private_parallelism_mul - Active sessions multiplier to deduce number of 
private strands

I'm sure Tanel or Steve may have greater insight.

Cheers
Connor

--- 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
> 


=====
Connor McDonald
Co-author: "Mastering Oracle PL/SQL - Practical Solutions"
ISBN: 1590592174

web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: connor_mcdonald@xxxxxxxxx

Coming Soon! "Oracle Insight - Tales of the OakTable"

"GIVE a man a fish and he will eat for a day. But TEACH him how to fish, 
and...he will sit in a boat and drink beer all day"

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