Message-ID: <52B46C3B.20902@optionshouse.com>
Date: Fri, 20 Dec 2013 10:11:39 -0600
From: Shaun Thomas <sthomas@optionshouse.com>
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To: Sev Zaslavsky <sevzas@gmail.com>, Sergey Konoplev <gray.ru@gmail.com>
CC: "pgsql-performance@postgresql.org" <pgsql-performance@postgresql.org>
Subject: Re: slow query - will CLUSTER help?
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On 12/20/2013 09:57 AM, Sev Zaslavsky wrote:

> There is a separate RAID-1 for WAL, another for tablespace and another
> for operating system.

I tend to stick to DB-size / 10 as a minimum, but I also have an OLTP 
system. For a more OLAP-type, the ratio is negotiable.

The easiest way to tell is to monitor your disk IO stats. If you're 
seeing a READ-based utilization percentage over 50% consistently, you 
need more RAM. On our system, we average 10% through the day except for 
maintenance and loading phases.

Of course, that's only for the current DB size. A good trick is to 
monitor your DB size changes on a daily basis, plot the growth 
percentage for a week, and apply compounding growth to estimate the size 
in three years. Most companies I've seen are on a 3-year replacement 
cycle, so that gives you how much you'll have to buy in order to avoid 
another spend until the next iteration.

For example, say you have a 800GB database, and it grows at 10GB per 
week, so that's 40GB per month. In three years, you could need up to:

800 * (1 + 40/800)^36 = 4632GB of space, which translates to roughly 
480-512 GB of RAM. You can probably find a comfortable middle ground 
with 240GB.

Of course, don't forget to buy modules in multiples of four, otherwise 
you're not taking advantage of all the CPU's memory channels. :)

-- 
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-676-8870
sthomas@optionshouse.com

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