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

From: John Clarke <>
Date: Thu, 3 Jan 2013 16:31:22 +0000
Message-ID: <>

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 <<>> Date: Wednesday, January 2, 2013 4:06 AM To: John Clarke <<>> Cc: "<>" <<>>, "<>" <<>> 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.?


On Mon, Dec 31, 2012 at 7:31 AM, John Clarke <<>> 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
Received on Thu Jan 03 2013 - 17:31:22 CET

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