Re: What MB/s is pulled from your buffer cache?
Date: Mon, 18 Feb 2013 19:48:33 +0200
Message-ID: <CAMHX9J+npW8UZbmnYG6HUZmGyKpu3y8rsO_BrVA9FqTfdcQqZQ_at_mail.gmail.com>
This metric was added in 11.2.0.3.
I wouldn't use this metric for anything practical really as it doesn't represent the actual bytes transferred. It's just the number of times a buffer get was done times the block size.
If you want to calculate it for some reason on earlier versions, then you can just multiply "session logical reads" with your DB block size. You're out of luck if you have multiple blocksizes.
But again, it doesn't show the actual number of bytes transferred/processed. You don't have to scan through the entire 8kB block every time you do a buffer get. Also, if a buffer is kept pinned and re-visited thousands of times under that pin, the logical reads* stats do not go up even though you might be accessing data there a lot of times in a loop...
And if anyone is thinking of writing a cool report which shows "physical read total bytes" divided by "logical read bytes from cache" then that's essentially buffer cache hit ration in disguise.
-- *Tanel Poder* Enkitec (The Exadata Experts) Training <http://blog.tanelpoder.com/seminar/> | Troubleshooting<http://blog.tanelpoder.com/> | Exadata<http://www.amazon.com/Expert-Oracle-Exadata-Apress/dp/1430233923> | Voicee App <http://voic.ee/> On Mon, Feb 18, 2013 at 3:40 PM, Martijn Bos <maboc_at_maboc.nl> wrote:Received on Mon Feb 18 2013 - 18:48:33 CET
> Hi,
>
> Interesting subject (at least to me)
>
> I'm on 11.2.0.1 on redhat(5).
> When executing this query I get no rows returned.
>
> Looking into v$statname I noticed this statistic is not there.
> Also the (online) documentation (11.2) didn't reveal much.
>
> So now I wonder where to get more information on this statistic.
> I find out more than once that the statistic name is (to me) not quite
> descriptive,
> so I need some documentation to determine how to 'calculate' the 'logical
> read bytes from cache' statistic for 11.2.0.1.
>
> So in short. 2 questions arise :
> 1 - Can anyone point me in the direction of some documentation
> 2 - Can anyone confirm that this statistic is available in 11.2.0.1 (or
> more presumably, that I'm mistaken)
>
>
> Best Regards,
> Martijn
>
>
>
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