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Re: Oracle and Raid setup

From: VC <boston103_at_hotmail.com>
Date: Sun, 12 Jun 2005 00:48:21 -0400
Message-ID: <jOKdnUsYkLm-qjHfRVn-pQ@comcast.com>


Hi,

<xhoster_at_gmail.com> wrote in message
news:20050609141630.752$66_at_newsreader.com...
> Frank van Bortel <frank.van.bortel_at_gmail.com> wrote:
[... skipped ...]

>> Why would a mirrored write be slower for writes? OK, as slow
>> as the slowest disk, but those differences can (should!) only
>> be marginal.
>
> If each write takes a random amount of time uniformly distributed
> between 0 and 1 (in whatever units of time would make sense), then the
> average wait for one write is 0.5, while the average wait for slower of
> two
> writes is 0.6667.

I wonder how you arrived at the number (0.6667)...

Let's assume that track seeks are distributed uniformly (each track, from a given position, is accessed with the same probability). Further, let's assume we have two *identical* disks. Then, the seek distance is a random variable Xr = min(X1, X2) for reads and Xw=max(X1,X2) for writes. Then, with another huge assumption of X1 and X2 being independent, it can be shown that the expected seek distance would be approx. 0.2*n for reads and 0.46*n where n is the number of tracks. For a single disk, both values are n/3. So, for reads we would have about 39% gain in performance and for writes about 38% loss.

VC

>That is a 33% penalty. I don't know if that qualifies
> as marginal or not. I don't know what a realistic actual distribution of
> write delays is, but I think uniform from 0 to 1 is not too outrageous of
> an assumption.
>
> Xho
>
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Received on Sat Jun 11 2005 - 23:48:21 CDT

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