ASH report - missing blocking session info

From: Vasu <vasudevanr_at_gmail.com>
Date: Wed, 23 Jan 2013 21:13:22 -0600
Message-ID: <CA+O6cLL-++Decw_sMQJorp_iyKgxf=f4z0ZK9MRWphhDf4fO=g_at_mail.gmail.com>

We have had perf-issues in our 11.2.0.3, Rac DB a while ago. As a post-mortem started looking into DBA_HIST_ACTIVE_SESS_HISTORY and ASH report for that period.
The scenario is : A session on instance-1 was blocked by a session from instance-2 (blocker) for nearly 40 minutes,

For the blocked session on instance-1, dba_hist_active_sess_history has  300+ records, a record for every 30 sec approximately , with all records having the same info : Waiting on enq wait, with blocking sesssion info :sid,serial# ,instance and status GLOBAL. (which is OK). Wow..I thought I was making a good progress, only a record away from the culprit.. but the blocker info is missing in DBA_HIST_ACTIVE_SESS_HISTORY,  and ash report too . :(

Quick glance of the 1st article thru google said 'only active sessions are sampled'.. so is it possible the blocker was idle after some DMLs (without issuing a commit)?
OR has any one come across 'missing session info ' for valid cases or possibly a bug?

Thanks
Vasu

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Received on Thu Jan 24 2013 - 04:13:22 CET