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Re: lots of waiting on 'db file parallel write'

From: Joel Garry <joel-garry_at_home.com>
Date: 27 Feb 2006 16:13:46 -0800
Message-ID: <1141085626.545737.206820@j33g2000cwa.googlegroups.com>


>Can you explain what you mean about v$bh and buffer thrashing and
>veritas housekeeping??

Well, let me first say this is just random shooting at symptoms, you should listen to Mladen.

What I mean about v$bh is, your dbwr are waiting on something, probably i/o. So, if there are things you can do to lessen i/o, they are likely to be good things. In a system as you have described, much is dependent on what everything else is doing. So, if you are reading things into the SGA (which the parallel reads don't do), then updating them, you have things happening in the SGA. Your users are continually pounding on the SGA, making oracle perform a lot of work moving things in and around. Your index scans especially will be flushing out the SGA, requiring more I/O (all those sequential waits). So, are you using multiple buffer pools? By placing the full-scanned tables into the recycle pool and pounded on indices in the keep pool, you lessen the number of tables moving stuff in and out of the SGA (and hopefully, some associated I/O), and in addition, keep Oracle from placing some moderately sized objects in the MRU end of the LRU list, shortening what needs to be searched through to get updates done in the SGA.

Since there tends to be cascading performance problems when you hit multiple bottlenecks, the slight change of using multiple buffer pools can have a dramatic effect, especially in a hybrid system. Some laugh at the implication of a silver bullet, but really it is just pretty basic tuning that happens to be on a performance inflection point.

As far as Veritas, it's been a long time since I worked with it, so I just have some hazy recollection of it sometimes just going bonkers with something like trying to catch up with some journalling under load. For what it's worth, which isn't much. But I seem to remember something about archiving logs just overwhelming everything due to their sequential write nature.

jg

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Received on Mon Feb 27 2006 - 18:13:46 CST

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