RE: gv$active_session_history.blocking_session
Date: Wed, 12 Jun 2019 10:03:37 +0000
Message-ID: <41398_1560333837_5D00CE0D_41398_1434_1_c70d4fa9ce544c4c92f244df120bce6f_at_vontobel.com>
Thank you, Tanel, for explaining the "idle blocker" ASH pattern. Your blog post is the only place I've seen this concept explicitly mentioned.
ash_wait_chains 0.4 indeed shows the "idle blocker":
_at_ash_wait_chains.sql session_id||':'||event2 "session_id=2231" "to_date('11.06.2019 07:08:00','dd.mm.yyyy hh24:mi:ss')" "to_date('11.06.2019 07:09:20','dd.mm.yyyy hh24:mi:ss')"
- Display ASH Wait Chain Signatures script v0.4 BETA by Tanel Poder ( http://blog.tanelpoder.com )
%This SECONDS AAS
------ ---------- ----------
WAIT_CHAIN
FIRST_SEEN LAST_SEEN
------------------- ------------------- 76% 37 .5
-> 2231:log file switch (private strand flush incomplete) -> [idle blocker ,736,37666 (oracle_at_svavt02a (LGWR))]
2019-06-11 07:08:30 2019-06-11 07:09:19
16% 8 .1
-> 2231:log file switch (private strand flush incomplete) -> 736:enq: CF - contention -> 757:db file single write
2019-06-11 07:08:41 2019-06-11 07:09:03
4% 2 0
-> 2231:log file switch (private strand flush incomplete) -> 736:control file sequential read
2019-06-11 07:08:37 2019-06-11 07:08:48
2% 1 0
-> 2231:log file switch (private strand flush incomplete) -> 736:ON CPU
2019-06-11 07:08:56 2019-06-11 07:08:56
2% 1 0
-> 2231:log file switch (private strand flush incomplete) -> 736:LGWR all worker groups -> [idle blocker ,778,64901 (oracle_at_svavt02a (LG00))]
2019-06-11 07:08:34 2019-06-11 07:08:34
Similarly to the problem described in your blog post, the idle LGWR was waiting on DBWR to complete. We can deduce this from the description of the event "log file switch (private strand flush incomplete)", see https://docs.oracle.com/database/121/REFRN/GUID-7F3D1BA7-2390-4911-B9E7-D2B26088F45D.htm#REFRN00583 :
"User sessions trying to generate redo, wait on this event when LGWR waits for DBWR to complete flushing redo from IMU buffers into the log buffer; when DBWR is complete LGWR can then finish writing the current log, and then switch log files."
In my case, the IO write performance slumped for a moment without any obvious reason - the load was insignificant. Next, I'm going to setup OS diagnostic ( https://github.com/brendangregg/dtrace-cloud-tools/blob/master/zfs/zfsslower.d ) to verify the write outliers from the OS perspective and start the analysis from there.
Best regards,
Nenad
https://nenadnoveljic.com/blog
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