RE: Someone remind me - SLOW LGWR - buffer flushes prevent new blocks being read in
- From: "Mark W. Farnham" <mwf@xxxxxxxx>
- To: <Christopher.Taylor2@xxxxxxxxxxxx>, <oracle-l@xxxxxxxxxxxxx>
- Date: Thu, 18 Oct 2012 17:11:31 -0400
Lots of possibilities:
A couple-tree likely candidates:
1) If LGWR falls far enough behind that you run short of log buffer space
then updates will hang until that is cured. (If logon auditing is on, that
can appear to be a complete hang to a casual glance.)
2) If LGWR and/or ARCH fall so far behind that the online redo capacitance
you have configured is full, about everything but ARCH hangs (filling the
archive destination is a classic way to make this happen. If LGWR can fall
this far behind on your system without ARCH being stopped, something is
probably dramatically wrong with your configuration. If bursty activity
sometimes causes ARCH to fall far enough behind so that online logs wrap,
you probably need more online log space, either via more or larger groups).
3) If you need your load of freeing buffers is very high combined with
occupied buffers being frequently updated, DBWR might have to wait for LGWR
before a buffer block and be written and become a candidate to be freed.
JL writes about this a bit in "Oracle Core" and Arup Nanda has a
presentation online various places about log buffer flushes piling up into
gc buffer busy waits with RAC. Craig S. makes a similar presentation that is
quite entertaining for a single instance, but I don't see it online at the
moment. I think Riyaj has also written about this.
So at steady load state if your need for new free buffer cache blocks
exceeds the rate buffers are freed, you'll eventually be waiting for free
buffers. If lgwr is slow enough to be the pacing task in freeing blocks,
then in that context "slow lgwr buffer flushes prevent new blocks being read
in."
In looking at why lgwr might be slow, it is useful to establish whether the
problem is lgwr getting sufficient CPU cycles at low enough latency to keep
up or your write rate once lgwr is actually writing something.
Good luck. That's the approximate shape of what I think you want to know.
For a detailed answer to a specific problem, we'll need the usual stuff to
help
-----Original Message-----
From: oracle-l-bounce@xxxxxxxxxxxxx [mailto:oracle-l-bounce@xxxxxxxxxxxxx]
On Behalf Of Christopher.Taylor2@xxxxxxxxxxxx
Sent: Thursday, October 18, 2012 3:25 PM
To: oracle-l@xxxxxxxxxxxxx
Subject: Someone remind me - SLOW LGWR - buffer flushes prevent new blocks
being read in
I was reading somewhere (and I can't put my hands on it now of course) where
when LGWR is slow, that it backs stuff up bad enough that blocks can't get
into the cache fast enough and will slow the whole database down (that's the
gist of it I think). When LGWR is exceptionally slow/hung.
Can someone point me in the right direction here?
Chris
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