> Adding drives will not let you get lower response times than the average seek
> time on your drives*. But it will let you reach that response time more often.

I believe your assertion is fundamentaly flawed.  Adding more drives
will not let you reach that response time more often.  All drives are
required to fill every request in all RAID levels (except possibly
0+1, but that isn't used for enterprise applicaitons).  Most requests
in OLTP require most of the request time to seek, not to read.  Only
in single large block data transfers will you get any benefit from
adding more drives, which is atypical in most database applications. 
For most database applications, the only way to increase
transactions/sec is to decrease request service time, which is
generaly achieved with better seek times or a better controller card,
or possibly spreading your database accross multiple tablespaces on
seperate paritions.

My assertion therefore is that simply adding more drives to an already
competent* configuration is about as likely to increase your database
effectiveness as swiss cheese is to make your car run faster.

Alex Turner

*Assertion here is that the DBA didn't simply configure all tables and
xlog on a single 7200 RPM disk, but has seperate physical drives for
xlog and tablespace at least on 10k drives.

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