On 6 May 2014 13:07, Michael Stone <mstone+postgres@xxxxxxxxx> wrote:
On Tue, May 06, 2014 at 11:13:42AM +0200, Johann Spies wrote: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.
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.
In that case I apologise for making the wrong assumption. People who are more experienced than me analyzed the logs told me that to their surprise the CPU' s were under pressure. I just assumed that the iowait was the problem having looked at the logs myself.
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.
I also thought this might be an overkill but I was not sure.
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.
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?
I understand your remark about the CPU in the light of my wrong assumption earlier, but I do not understand your remark about the RAM. The fact that temporary files of up to 250Gb are created at times during complex queries, is to me an indication of too low RAM.
Question: How do I dedicate a partition to indexes? Were do I configure PostgreSQL to write them in a particular area?
Regards
Johann
Johann
