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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