Re: gc buffer busy and rac testing

From: Jonathan Lewis <>
Date: Thu, 8 Jan 2009 11:54:01 -0000
Message-ID: <>

"helter skelter" <> wrote in message news:gk4ors$qae$
> Hi,
> I'am testing 2 node RAC but I haven't got too much experince with this
> technology and I am little puzzled after test results.
> I used Swingemch (order_entry test) and when I ran this test on 2 nodes
> RAC performance is much lower then on only one node of this cluster.
> 2 nodes - 324
> 1 node - 473
> 2 nodes - 17571
> 1 node - 25272
> When I look at waits I see (2 node test) that 60% are "gc buffer busy".
> Isn't it too much ? any advices what more can I check?
> Tablespace is localy managed, with auto segment space management and
> system allocation type. Interconnect on dedicated 1GB ethernet. Oracle
> (standard edition) on RHEL5 with asm.
> thakns

"gc buffer busy" is the RAC equivalent of "buffer busy waits" when you have multiple nodes that (usually) want to modify the same blocks at the same time.

In your case, with order entry, the fact that you are using ASSM probably stops this from happening in the tables, but you will have primary key indexes on things like ORDERS and ORDER_LINES which are probably sequence-based, and the "right hand" block from these indexes is probably suffering severe competition as it hammers back and fore between the two nodes.

There may also be problems with the block holding the sequence definitions, but in this case you'd also see waits on the SQ enqueue.

Check the AWR reports (or statspack running at level 7 if you're not licensed for AWR) and you will find:

"SQL ordered by cluster wait time" - this will probably show you the most significant SQL relating to the problem. Then check the segment statistics "'Segments by Global Cache Buffer Busy Waits " this will tell you which objects are hit hardest.

If the object do happen to be sequence-based primary key indexes, then you may be able to address the issue simply by rebuilding the indexes as REVERSE indexes. (This does have some side-effects, including the potential for increasing the size of the buffer cache to help Oracle maintain
the caching benefit it had on this index before reversal).


Jonathan Lewis

Author: Cost Based Oracle: Fundamentals

The Co-operative Oracle Users' FAQ
Received on Thu Jan 08 2009 - 05:54:01 CST

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