Re: Flashback log file sync
Date: Sat, 12 May 2012 23:41:21 +1000
Message-ID: <jolp8p$ci9$1_at_dont-email.me>
Mladen Gogala wrote,on my timestamp of 12/05/2012 3:37 PM:
> This is only to show what a hefty performance penalty is exacted by
> having flashback turned on.
A few weeks ago an Oracle "expert" insinuated I was a "bad dba" for refusing to use flashback in our production DW while testing new BI code releases.
I gave up on it a long time ago after watching the performance hit on some typical loads. Instead, I provide an exact copy of production - down to the same SAN LUN types and numbers - to break as they please. Fun and games to keep the optimizer behaving the same in both instances, but I can live with that.
Flashback has to read and then write the data somewhere, somehow. And when it is written in the flashback area, it is potentially from multiple tablespaces into one set of devices. So much for I/O spreading. Not easily tunable - I don't think using "hidden" parameters is a reliable long term tuning idea - and I/O to flashback includes data and indexes.
Of course: there are special cases where it is useful and doesn't terribly harm performance. On standby dbs for testing DR, for example. But on a DW? And with large loads and data cleansing - typical ETL cycle - happening all day?
No, flashback can't possibly, ever, cause a performance hit under typical DW process loads!... Ah well, how "bad dba" of me to actually care about performance instead of features. What can I say. Received on Sat May 12 2012 - 08:41:21 CDT