I'm with LSC--all you know is that the same session ID is connected to the database multiple times, not necessarily through a different instance. Many applications (including SQL*Plus) will open multiple database sessions for certain operations and that's the expected behavior. I believe that those additional sessions (at least in the case of SQL*Plus) are made using the same database connection, so they should land on the same instance. In addition to gv$session.inst_id, you might also need to check out the columns gv$session.failover_type and failover_method to see if those "extra" sessions are the result of a preconnect session. Dan ----- Original Message ---- From: LS Cheng <exriscer@xxxxxxxxx> To: staywithpin@xxxxxxxxx Cc: oracle-l <oracle-l@xxxxxxxxxxxxx> Sent: Thursday, September 27, 2007 1:23:55 AM Subject: Re: The same session run on both instances (RAC), is it normal? how about group by inst_id...? thanks -- LSC On 9/27/07, qihua wu <staywithpin@xxxxxxxxx > wrote:We have a two nodes RAC, the strange thing is that many sessions run on > both nodes. select sid from gv$session group by sid having count(*)>1 they are lots of rows return. My understanding for load balance is: if sessions on instance 1 are too many, then new session will be connected to instance 2. But if at the beginning there were100 sessions on both inst1 and inst2, after all the sessions on inst2 are completed, no existing session on instance1 will go to instance2. But the fact of the above query told me some sessions run on both instance. How can this be true?