Date: Tue, 6 May 2014 07:07:14 -0400
From: Michael Stone <mstone+postgres@mathom.us>
To: pgsql-performance@postgresql.org
Subject: Re: Specifications for a new server
Message-ID: <20140506110714.GA9155@mathom.us>
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On Tue, May 06, 2014 at 11:13:42AM +0200, Johann Spies wrote:
>Analysis or the SAR-logs showed that there were too much iowait in the CPU's on
>the old system which has a lower spec CPU than the ones considered for the new
>system.

iowait means the cpu is doing nothing but waiting for data from the 
disk. buying faster cpus means that they will be able to spend more time 
waiting for data from the disk. you'd probably get much better bang for 
the buck upgrading the storage subsystem than throwing more money at 
cpus.

>We are looking possibly the following hardware:
>
>CPU: 2 x  Ivy Bridge 8C E5-2667V2 3.3G 25M 8GT/s QPI - 16 cores
>RAM: 24 x 32GB DDR3-1866 2Rx4 LP ECC REG RoHS  - 768Gb
>
>with enough disk space - about 4.8 Tb on RAID 10.
>My question is about the possible advantage and usage of SSD disks in the new
>server.  

>At the moment I am considering using 2 x 200GB SSD' s for a separate
>partion for temporary files and 2 x 100GB for the operating system.

If you're talking about SSDs for the OS, that's a complete waste; there 
is essentially no I/O relating to the OS once you've booted.

>So my questions:
>
>1. Will the SSD's in this case be worth the cost?
>2.  What will the best way to utilize them in the system?

The best way to utilize them would probably be to spend less on the CPU 
and RAM and more on the storage, and use SSD either for all of the 
storage or for specific items that have a high level of I/O (such as the 
indexes). Can't be more specific than that without a lot more 
information about the database, how it is utilized, and what's actually 
slow.

Mike Stone


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