RE: Large discrepancy between 'log file parallel write' and 'db file parallel write' times
Date: Tue, 8 Jan 2013 09:15:39 +0000
Message-ID: <DUB114-W482C774ADFD7A92977290EB7240_at_phx.gbl>
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_at_tanelpoder.com
To: john.clarke_at_centroid.com
CC: puravc_at_gmail.com; mccmx_at_hotmail.com; oracle-l_at_freelists.org
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_at_centroid.com> 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.
--
http://www.freelists.org/webpage/oracle-l
Received on Tue Jan 08 2013 - 10:15:39 CET