Re: Large discrepancy between 'log file parallel write' and 'db file parallel write' times
Date: Thu, 3 Jan 2013 16:31:22 +0000
Message-ID: <5EE1E2A57A88A740B5E3B5F945EFCDF40B23F818_at_ORD2MBX01F.mex05.mlsrvr.com>
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.
Matt responded to the list this morning and mentioned that Smart Flash Logging played a role in the behavior he saw on his environment, which makes sense as well.
- John
From: Purav Chovatia <puravc_at_gmail.com<mailto:puravc_at_gmail.com>> Date: Wednesday, January 2, 2013 4:06 AM To: John Clarke <john.clarke_at_centroid.com<mailto:john.clarke_at_centroid.com>> Cc: "mccmx_at_hotmail.com<mailto:mccmx_at_hotmail.com>" <mccmx_at_hotmail.com<mailto:mccmx_at_hotmail.com>>, "oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>" <oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>> Subject: Re: Large discrepancy between 'log file parallel write' and 'db file parallel write' times
Hi John,
Is the behaviour specific to exadata or that is fully/partially true in case of ASM too? Does any kind of background I/O queue exist on non-exadata but ASM (RAC/non-RAC) based env.?
Thanks
On Mon, Dec 31, 2012 at 7:31 AM, John Clarke <john.clarke_at_centroid.com<mailto:john.clarke_at_centroid.com>> wrote: Redo log and control file writes are considered "high priority" I/O consumers and get lumped into the high priority background I/O queue, so cellsrv will satisfy these requests at a higher priority than anything else. DBWR writes (which your db file parallel write waits is associated with) are prioritized the same as user I/O, relatively speaking, and a handful of other background-related writes and lower than log file/control file writes.
Under heavy I/O load and saturation, this behavior should be what you may expect.
- John
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