Skip site navigation (1) Skip section navigation (2)

Re: weird performances problem

From: Guillaume Smet <guillaume(dot)smet(at)openwide(dot)fr>
To: Ron <rjpeace(at)earthlink(dot)net>
Cc: pgsql-performance(at)postgresql(dot)org
Subject: Re: weird performances problem
Date: 2005-11-22 14:26:43
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
Ron wrote:
> If I understand your HW config correctly, all of the pg stuff is on the 
> same RAID 10 set?

No, the system and the WAL are on a RAID 1 array and the data on their 
own RAID 10 array.
As I said earlier, there's only a few writes in the database so I'm not 
really sure the WAL can be a limitation: IIRC, it's only used for writes 
isn't it?
Don't you think we should have some io wait if the database was waiting 
for the WAL? We _never_ have any io wait on this server but our CPUs are 
still 30-40% idle.

A typical top we have on this server is:
  15:22:39  up 24 days, 13:30,  2 users,  load average: 3.86, 3.96, 3.99
156 processes: 153 sleeping, 3 running, 0 zombie, 0 stopped
CPU states:  cpu    user    nice  system    irq  softirq  iowait    idle
            total   50.6%    0.0%    4.7%   0.0%     0.6%    0.0%   43.8%
            cpu00   47.4%    0.0%    3.1%   0.3%     1.5%    0.0%   47.4%
            cpu01   43.7%    0.0%    3.7%   0.0%     0.5%    0.0%   51.8%
            cpu02   58.9%    0.0%    7.7%   0.0%     0.1%    0.0%   33.0%
            cpu03   52.5%    0.0%    4.1%   0.0%     0.1%    0.0%   43.0%
Mem:  3857224k av, 3307416k used,  549808k free,       0k shrd,   80640k 
                    2224424k actv,  482552k in_d,   49416k in_c
Swap: 4281272k av,   10032k used, 4271240k free                 2602424k 

As you can see, we don't swap, we have free memory, we have all our data 
cached (our database size is 1.5 GB).

Context switch are between 10,000 and 20,000 per seconds.

> This concept works for other tables as well.  If you have a tables that 
> both want services at the same time, disk arm contention will drag 
> performance into the floor when they are on the same HW set.
> Profile your HD access and put tables that want to be accessed at the 
> same time on different HD sets.  Even if you have to buy more HW to do it.

I use iostat and I can only see a little write activity and no read 
activity on both raid arrays.


In response to


pgsql-performance by date

Next:From: Guillaume SmetDate: 2005-11-22 14:37:52
Subject: Re: weird performances problem
Previous:From: Alan StangeDate: 2005-11-22 14:26:38
Subject: Re: Hardware/OS recommendations for large databases (

Privacy Policy | About PostgreSQL
Copyright © 1996-2017 The PostgreSQL Global Development Group