Re: severe performance issues when flashback is on
- From: Laimutis.Nedzinskas@xxxxxx
- To: exriscer@xxxxxxxxx
- Date: Thu, 20 Oct 2011 10:43:17 +0300
flashback is sloooow.
One aspect (LOB related) I blogged myself:
http://laimisnd.wordpress.com/2011/03/25/lobs-and-flashback-database-performance/
That one and the orther aspects are very honestly written in this:
Flashback Database Best Practices & Performance [ID 565535.1]
Example:
"For batch direct load workloads, flashback can cause as much as 30%
overhead in Oracle 10g. In Oracle 11g, users can experience 2%-30%
flashback overhead for batch direct workloads. "
I second that - recently we switched off flashback on one DW-alike database
cause after tuning AWR was showing flashback events at the top.
---------------------------------------------------------------------------------
Please consider the environment before printing this e-mail
From: LS Cheng <exriscer@xxxxxxxxx>
To: Oracle Mailinglist <oracle-l@xxxxxxxxxxxxx>
Date: 2011.10.19 21:22
Subject: severe performance issues when flashback is on
Hi
I am currently reviweing some batch processes, most of them are INSERT...
SELECT, in production environment where flashback is enabled the speed is
quite slow compared with QA environment where flashback is off.
I traced one single INSERT SELECT statement and noticed that there are
always tons of physical reads (db file sequential read) in the table
segment
where I am inserting when flashback is on. Even I have a pretty lareg
buffer
cache and running the same insert more than once (so the second execution
can use buffer reads), the same test with flashback off works as expected.
I
dont see any reasoning why db file sequential reads always occur.
I expected overheads with flashback but it just seems too much overhead, I
did the same test in 11.2.0.2 and 10.2.0.5 and both shows similar
behaviour.
Am I missing anything?
Thanks
a simple test when flashback is on with 11.2.0.2 Linux x64, these tables
are
plain tables, no indexes:
create table t1
as
select * from dba_source
union all
select * from dba_source
union all
select * from dba_source
union all
select * from dba_source
union all
select * from dba_source
union all
select * from dba_source;
create table t2
as
select * from t1
where 1 = 0;
insert into t2
select * from t1;
Elapsed: 00:00:25.45
Statistics
----------------------------------------------------------
989 recursive calls
165874 db block gets
41894 consistent gets
36232 physical reads
121920148 redo size
841 bytes sent via SQL*Net to client
788 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
10 sorts (memory)
0 sorts (disk)
899628 rows processed
10046 tracefile
call count cpu elapsed disk query current
rows
------- ------ -------- ---------- ---------- ---------- ----------
----------
Parse 2 0.00 0.00 0 1
0 0
Execute 2 11.32 52.25 54331 86771 327219
1799256
Fetch 0 0.00 0.00 0 0
0 0
------- ------ -------- ---------- ---------- ---------- ----------
----------
total 4 11.32 52.26 54331 86772 327219
1799256
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total
Waited
---------------------------------------- Waited ----------
------------
db file sequential read 36759 1.06
30.21* --> this slows the insert*
log buffer space 4 0.47
0.60
flashback buf free by RVWR 17 0.99
4.98
log file switch (checkpoint incomplete) 6 2.33
4.72
log file switch completion 4 0.11
0.30
SQL*Net message to client 2 0.00
0.00
SQL*Net message from client 2 0.00
0.00
db file scattered read 212 0.53
3.06
-- second execution
Elapsed: 00:00:26.86
Statistics
----------------------------------------------------------
189 recursive calls
161345 db block gets
44974 consistent gets
18099 physical reads
121684968 redo size
844 bytes sent via SQL*Net to client
788 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
899628 rows processed
without flashback:
Elapsed: 00:00:08.12
Statistics
----------------------------------------------------------
831 recursive calls
146925 db block gets
41943 consistent gets
2 physical reads
121923948 redo size
830 bytes sent via SQL*Net to client
788 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
899628 rows processed
Elapsed: 00:00:10.65
Statistics
----------------------------------------------------------
180 recursive calls
143188 db block gets
44656 consistent gets
0 physical reads
121702204 redo size
839 bytes sent via SQL*Net to client
788 bytes received via SQL*Net from client
3 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
899628 rows processed
10046 tracefile
call count cpu elapsed disk query current
rows
------- ------ -------- ---------- ---------- ---------- ----------
----------
Parse 2 0.00 0.00 0 1
0 0
Execute 2 3.83 18.74 2 86469 290113
1799256
Fetch 0 0.00 0.00 0 0
0 0
------- ------ -------- ---------- ---------- ---------- ----------
----------
total 4 3.84 18.74 2 86470 290113
1799256
Elapsed times include waiting on following events:
Event waited on Times Max. Wait Total
Waited
---------------------------------------- Waited ----------
------------
Disk file operations I/O 2 0.00
0.00
db file sequential read 2 0.00
0.00
log buffer space 11 0.78
2.90
log file switch completion 6 0.40
0.87
log file switch (checkpoint incomplete) 8 3.70
10.56
SQL*Net message to client 2 0.00
0.00
SQL*Net message from client 2 0.00
0.00
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