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Re: How fast should an IO be on a SYMMETRIX?

From: Peter J. Holzer <hjp-usenet2_at_hjp.at>
Date: Sun, 2 Nov 2003 11:22:36 +0100
Message-ID: <slrnbq9mnc.l5k.hjp-usenet2@teal.hjp.at>


On 2003-11-02 06:04, Sal L <member46676_at_dbforums.com> wrote:
> This is rows from the tkprof output:
>
> call count cpu elapsed disk query current
> Fetch 33 0.06 2.50 254 736 0
>
>
> Well, but VC got the following numbers on his home PC with IDE disk:
>
> call count cpu elapsed disk query current
> Fetch 2 0.27 0.54 375 376 0

Is that the same query on an identical database (same tables, same data, etc.?)

> Based upon method of Jonathan Lewis the rate of IDE disk is 1.44 ms per
> read.
>
> Who can explain me why IDE disk in 7 times faster than Symmetrix ?

My guess is that the query on the home PC hit mostly blocks which were already in the operating system's cache, while the query on the HP machine hit mostly blocks which had to be read from disk. One reason for that may be that the DB on the HP is on raw devices which are never cached by the OS. They should be in the symmetrix cache, however, if the same query is repeated.

> And what is magic value of 10 ms for disk (may be this one is defined by
> disk seek time) ?

Yes.

> Otherwise, assume that the block size is 8K and as Symmetrics timing
> is 10 ms that transfer rate will be equal 800 KB/s.
>
> It seems very slow.

Yes. But it's about correct for reading random blocks. Readings disks sequentially is a lot faster than reading them in random order.

> May be the best solution is to move database from HP-UX server and
> Symmetrix storage to laptop or home PC with IDE disk ?

:-)

        hp

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Received on Sun Nov 02 2003 - 04:22:36 CST

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