Re: Disk I/O

rainer wrote:
>
> Hello,
>
> we are using a 9.2.0.5 DB on a Windows Advanced Server machine having 8 *
> 1.8 GHz CPUs, 8GB of MM.
> The database is used as well as DW as also as OLTP system. Data is stored on
> three Clariion RAID5 / fiber channel disk systems.
> When monitoring the database I can see lots of disk waits, which is expected
> when considering the relation CPU performance / number of disks.
> However, when looking at the Clariion performance monitor, I see between 40
> and 80 IO requests / sec and between 0 and 4 queued IO requests. The queue
> size does not raise during regular DB operation.
> When doing additional disk operations via OS, as e.g. copying a file, IO
> requests raise up to 600 / sec.
>
> The question is: Why is the DB waiting for IO, while the disks seem not to
> work at their limits?

Pick up a copy of "Computer Architecture: A Quantitative Approach", Hennesey. It shows numerous examples of where when you start asking a disk subsystem to give you lots of small size IO's, then you can get queueing whilst all the utilisation figures don't seem at all bad...

hth
connor

-- 
Connor McDonald
Co-author: "Mastering Oracle PL/SQL - Practical Solutions"
Co-author: "Oracle Insight - Tales of the OakTable"

web: http://www.oracledba.co.uk
web: http://www.oaktable.net
email: connor_mcdonald_at_yahoo.com


"GIVE a man a fish and he will eat for a day. But TEACH him how to fish,
and...he will sit in a boat and drink beer all day"

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