Re: disks for redo logs

From: Noons <>
Date: Fri, 13 Feb 2009 23:37:08 +1100
Message-ID: <gn3p8j$sdb$>

joel garry wrote,on my timestamp of 13/02/2009 5:51 AM:

> I've seen this advice before, but haven't done anything, since, hey,
> if the thing is I/O bound, wouldn't making I/O initiates more often
> make it worse?

No, not more often. Jacking up the priority of lgwr does nothing to change the frequency or rate of lgwr IO activation: that is not determined by OS priority.

What it does is this: *if* or *when* the lgwr is posted to wake up and start an IO, then it does so without having to sit in a queue with other processes at same priority who might also be waiting for CPU time slot.

Once the IO is started, lgwr simply goes back to sleep waiting for IO completion or next activation, without waiting for the end of time slot.

Because lgwr doesn't have to do much other than start a write of buffers to disk, it won't consume much CPU before it suspends waiting for completion. As such, the higher priority does not significantly affect other processes.

Classical OS scheduling theory: IO-bound processes get a higher priority so they can quickly start their short CPU demand to initiate an IO, with the least possible delay, and then go back to sleep.

> documented limits. Needless to say, the latter has gotten much worse
> with scaling and going to fewer, faster Itanium processors.

Bingo! It never fails. Unreal...

> so ask for Oracle trace files. Which of course don't say anything at
> all because the program isn't asking the database for anything when it
> is cpu bound. I wouldn't want to make that worse fighting background
> processes.

Ah, OK: not much redo log generation by all that in-memory crap. Then you're OK. Here, I'm seeing redos sized at 200MB switching every 5 seconds on a good day, for periods in excess of one hour. And they gotta be archived...

> Unfortunately, some end-of-month processing has been converted to use
> those misfeatures in the new version, so I get a lot of
> "arewethereyetarewethereyetarewethereyettheoldsystemwasfaster!" Users
> having forgotten not so long ago when it took days so they called me
> in... 8 years already? Wore out a whole Chrysler...

Yikes! Man! How many miles are you doing daily? When I was with the air force, at one stage I was doing 55 each way... Received on Fri Feb 13 2009 - 06:37:08 CST

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