RE: Large discrepancy between 'log file parallel write' and 'db file parallel write' times

  • From: Matt McClernon <mccmx@xxxxxxxxxxx>
  • To: <tanel@xxxxxxxxxxxxxx>, <john.clarke@xxxxxxxxxxxx>
  • Date: Tue, 8 Jan 2013 09:15:39 +0000

Hi Tanel,
I've always wondered whether log file writes are really sequential I/O.  for 
example on an Exadata platform the redo logs are in the RECO disk group which 
is carved from the same set of disks as the DATA disk group.   So when a log 
file parallel write hits a disk, wont the head have potentially moved to 
service a DBWR write (or a sort write) since the last log file write, 
effectively making them 'random writes'.
Matt  

Date: Sun, 6 Jan 2013 12:43:54 +0800
Subject: Re: Large discrepancy between 'log file parallel write' and 'db file 
parallel write' times
From: tanel@xxxxxxxxxxxxxx
To: john.clarke@xxxxxxxxxxxx
CC: puravc@xxxxxxxxx; mccmx@xxxxxxxxxxx; oracle-l@xxxxxxxxxxxxx

Also note that the "log file parallel write" is not really random IO, compared 
to a "db file parallel write" which may write hundred(s) of buffers into random 
locations in a single vector IO call. That makes a difference even on 
non-exadata without IORM too (especially if there's no write-cache for DBWR 
writes or the storage array cache destaging can't keep up with the write 
workload).


-- Tanel PoderNew Online Training!http://blog.tanelpoder.com/seminar



On Fri, Jan 4, 2013 at 12:31 AM, John Clarke <john.clarke@xxxxxxxxxxxx> wrote:

The behavior I was describing was specific to IORM & Exadata I/Os.  To my 
knowledge there isn't a direct parallel to this for ASM in general, but I'm 
open to be educated ...


With ASM without Exadata, background I/O is queued according to your async I/O 
configuration (or synchronously depending on O/S and init.ora settings), but 
the operating system or Oracle wouldn't interject any prioritization algorithm 
for LGWR I/Os vs DBWR I/Os.

                                          
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