Re: extremly low memory usage - Mailing list pgsql-performance

At 02:53 PM 8/20/2005, Jeremiah Jahn wrote:
>On Fri, 2005-08-19 at 16:03 -0500, John A Meinel wrote:
> > Jeremiah Jahn wrote:
> > > On Fri, 2005-08-19 at 12:18 -0500, John A Meinel wrote:
> > >
><snip>
> >
> > > it's cached alright. I'm getting a read rate of about 150MB/sec. I would
> > > have thought is would be faster with my raid setup. I think I'm going to
> > > scrap the whole thing and get rid of LVM. I'll just do a straight ext3
> > > system. Maybe that will help. Still trying to get suggestions for a
> > > stripe size.
> > >
> >
> > I don't think 150MB/s is out of the realm for a 14 drive array.
> > How fast is time dd if=/dev/zero of=testfile bs=8192 count=1000000
> >
>time dd if=/dev/zero of=testfile bs=8192 count=1000000
>1000000+0 records in
>1000000+0 records out
>
>real    1m24.248s
>user    0m0.381s
>sys     0m33.028s
>
>
> > (That should create a 8GB file, which is too big to cache everything)
> > And then how fast is:
> > time dd if=testfile of=/dev/null bs=8192 count=1000000
>
>time dd if=testfile of=/dev/null bs=8192 count=1000000
>1000000+0 records in
>1000000+0 records out
>
>real    0m54.139s
>user    0m0.326s
>sys     0m8.916s
>
>
>and on a second run:
>
>real    0m55.667s
>user    0m0.341s
>sys     0m9.013s
>
>
> >
> > That should give you a semi-decent way of measuring how fast the RAID
> > system is, since it should be too big to cache in ram.
>
>about 150MB/Sec. Is there no better way to make this go faster...?
Assuming these are U320 15Krpm 147GB HDs, a RAID 10 array of 14 of
them doing raw sequential IO like this should be capable of at
  ~7*75MB/s= 525MB/s using Seagate Cheetah 15K.4's,  ~7*79MB/s=
553MB/s if using Fujitsu MAU's, and ~7*86MB/s= 602MB/s if using
Maxtor Atlas 15K II's to devices external to the RAID array.

_IF_ the controller setup is high powered enough to keep that kind of
IO rate up.  This will require a controller or controllers providing
dual channel U320 bandwidth externally and quad channel U320
bandwidth internally.  IOW, it needs a controller or controllers
talking 64b 133MHz PCI-X, reasonably fast DSP/CPU units, and probably
a decent sized IO buffer as well.

AFAICT, the Dell PERC4 controllers use various flavors of the LSI
Logic MegaRAID controllers.  What I don't know is which exact one
yours is, nor do I know if it (or any of the MegaRAID controllers)
are high powered enough.

Talk to your HW supplier to make sure you have controllers adequate
to your HD's.

...and yes, your average access time will be in the 5.5ms - 6ms range
when doing a physical seek.
Even with RAID, you want to minimize seeks and maximize sequential IO
when accessing them.
Best to not go to HD at all ;-)

Hope this helps,
Ron Peacetree