Re: Hot index block split on a very busy table--what is the impact

If a session is waiting on another session to complete an index block split, the wait will probably be a TX wait - probably mode 6, but possibly mode 4.

The worst case scenarios arise when another (empty) leaf block has to be taken off the free list, and out of the index structure at the same time - if the leaf block is then recognised as inappropriate (for reasons that I don't fully understand) the split is rolled back and another empty leaf block tested. If you are unlucky you may have many leaf blocks tested and rejected in rapid sequence - leading to lots of start/stops on the TX waits.

The problem arises if you have a system that regularly empties a number of leaf blocks (few hundred rows deleted in one hit, say), and then rapidly inserts more data.

The fact that you are seeing latch activity as the threat is possible because of the nature of your client - I'd guess it's something like a web app using .net. When the database "slows down" it starts more sessions - which hit the shared pool kicking out load so objects, so the system slows down more, so the client starts up more sessions, which hit the shared pool ...

Checks:

    v$sysstat shows branch block and leaf block splits -     but you problem may be a single attempt at a branch     block split that keeps failing - so the stats won't be     high.

    v$library_cache, v$sgastat - around the event, you     should see lots of reloads, and lots of free memory     appearing.

Damage limitation:

    If my description of the table/index activity is correct,     do an index coalesce after each big delete. Cleans up     the mess and takes empty leaf blocks out of the structure

    If my description of the client is correct, configure it to     start with more processes, and launch new processes     at a slower rate - then increase the shared_pool_reserved     to be able to hold about 25KB for every process you     might start, with a few MB spare.

Regards

Jonathan Lewis
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• Original Message ----- From: "Zhu,Chao" <zhuchao_at_gmail.com> To: <oracle-l_at_freelists.org> Sent: Wednesday, August 30, 2006 2:53 PM Subject: Hot index block split on a very busy table--what is the impact

> hi, all,
> We have a few database with very high concurrent access busy tables,
> that some indexes of the busy tables could be accessed 1.5K-4K times per
> second. (PK lookup)
> I am wondering , when such index got block split (at the root, or branch
> level), what will be the impact on the system.
> Index block split is said to be an expensive operation, during the block
> split on branch/root block, what the other sessions that is doing select
> based on this index, be waiting on? There is a wait event named: Index
> Block Split with p1,p2,p3 pointing to block address, level.
> Document "Description of oracle 7 Wait event and enqueue" says it will
> yield CPU.
> We have a few production incident when load suddenly jump from 10, to
> several hundred, and during that time nothing can be done. From our
> monitoring tools, it is mainly "latch free" contention inside oracle, and it
> is cache buffer chains. The load spike matches the oracle description of
> block split, but the wait event does not match. And because the load spike
> time is very short, we can't capture the actual wait event/p1,p2,p3 during
> the exact time of the load spike time.
> Anyone have similar problem/insight into this issue?
> Is there some v$ view to track, (v$segstat does not) have log about which
> index go through how many time block split? Or any other effecitve way?
>
>
> --
> Regards
> Zhu Chao
> www.cnoug.org
>

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