Re: Oracle 9i Latching Issue

On Mar 28, 2:42 pm, "fitzjarr..._at_cox.net" <fitzjarr..._at_cox.net> wrote:
> On Mar 28, 8:18 am, Johne_uk <edg..._at_tiscali.co.uk> wrote:
>
>
>
>
>
> > Hi,
>
> > I'm a beginner Oracle DBA who is facing a performance issues on a 9i
> > instance running on Solaris. The db
> > instance hosts a number of web applications including in-house apps
> > and oracle apps such as Discoverer.
>
> > For the past month there have been periods where the db has slowed
> > down to an almost unusable state. I've
> > looked at various options including reusable sql in the shared pool
> > etc but, using statspack, I've finally manged to find a corellation
> > whereby during slow periods the latch free waits % increases
> > dramatically.
>
> > As there are a multitude of apps running from this instance I need to
> > try and pinpoint whichs app(s) are generating the latch free waits.
>
> > My findings. When all is well my Latch Hit % is 99.xx which is normal.
> > During slowdown earlier today this dropped down to 85% indicating an
> > issue.
>
> > Digging deeper into this the primary culprit for this latch activity
> > seems to be with cache buffer chain latches which were experiencing a
> > 25% miss rate.
>
> > Once the db speeded up latches reverted back to being insignificant.
> > My suspicion is that the issue is being
> > caused by Discoverer sessions as the sessions always features
> > predominantly in statspack ouput with large
> > volumes of buffer gets during slowdown.
>
> > However, what I now need is some input as regards how to find the
> > offending sessions that are causing these
> > excessive latch waits times. I'd appreciate some assistance from the
> > user community on this as I'm on a steep
> > learning curve for my experience level.
>
> > It would almost be impossible to rewrite the sql in any apps but I may
> > have the option to remove any offending
> > apps to another db server as opposed to our primary prod instance.
>
> > It should also be pointed out that the issue is not caused by rogue
> > sessions thrashing the db, as activity can be minimal during slowdown.
>
> > Thanks in anticipation
> > John
>
> Usually such contention is caused by simultaneous access to a given
> object (often a single block of that object).  It may be an index leaf
> block causing this contention; you might try rebuilding the suspect
> index as a reverse-key index and see if that spreads out the access
> (keys which were adjacent in a standard index likely won't be in a
> reverse-key index and will lighten the load on a single index leaf
> block).  If it's the root block of that index then there isn't much
> you can do other than rebuild the table as a partitioned object and
> use local indexes.
>
> You might look at Metalink Note 163424.1 to investigate the 'hot
> block' angle.
>
> David Fitzjarrell- Hide quoted text -
>
> - Show quoted text -

Thanks I'll look at that Metalink Note as I'm fairly of the mind that it could be linked to a hotspot in the buffer cache.

cheers Received on Fri Mar 28 2008 - 10:32:04 CDT