Message-ID: <4BDB5597.3040306@2ndquadrant.com>
Date: Fri, 30 Apr 2010 18:11:35 -0400
From: Greg Smith <greg@2ndquadrant.com>
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To: Scott Whitney <swhitney@journyx.com>
CC: Scott Marlowe <scott.marlowe@gmail.com>, pgsql-admin@postgresql.org,
	Anj Adu <fotographs@gmail.com>, john@journyx.com
Subject: Re: more 10K disks or less 15K disks
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Scott Whitney wrote:
> On the 10k vs 15k rpm disks, there's a _lot_ to be said about that. I don't want to start a flame war here, 
> but 15k versus 10k rpm hard drives does NOT equivocate to a 50% increase in read/write times, to say 
> the VERY least.
>   

Your characterization is correct were there only the drives involved 
here, so no flames on your raw data.

Once you've introduced a battery-backed write cache into the mix, this 
whole area becomes impossible to compute that way though, and it was 
that context I was commenting from at least.  Those are good at turning 
random I/O into something more like sequential, as well as reducing the 
number of times you pay for rotational latency in several common 
database operations.  The effective impact is to significantly narrow 
the difference between drives where the seek and rotation time are the 
main differences in a database context--even though the worst-case IOPS 
doesn't really change.  IOPS is an interesting number to compute, but 
real-world database performance isn't linearly correlated with it.  
Maybe if your workload consists mainly of random, uncached index scans 
system performance will scale just like IOPS, but that's pretty uncommon.

[Rant about making sure not to drink the storage vendor IOPS Kool-Aid 
deleted]

-- 
Greg Smith  2ndQuadrant US  Baltimore, MD
PostgreSQL Training, Services and Support
greg@2ndQuadrant.com   www.2ndQuadrant.us