On Fri, 28 Jan 2005 11:54:57 -0500, Christopher Weimann
> On 01/28/2005-10:59AM, Alex Turner wrote:
> > At this point I will interject a couple of benchmark numbers based on
> > a new system we just configured as food for thought.
> >
> > System A (old system):
> > Compaq Proliant Dual Pentium III 933 with Smart Array 5300, one RAID
> > 1, one 3 Disk RAID 5 on 10k RPM drives, 2GB PC133 RAM.  Original
> > Price: $6500
> >
> > System B (new system):
> > Self Built Dual Opteron 242 with 2x3ware 9500S-8MI SATA, one RAID 1
> > (OS), one 4 drive RAID 10 (pg_xlog), one 6 drive RAID 10 (data) on 10k
> > RPM Raptors, 4GB PC3200 RAM.  Current price $7200
> >
> > System A for our large insert job: 125 minutes
> > System B for our large insert job: 10 minutes.
> >
> > There is no logical way there should be a 12x performance difference
> > between these two systems,  maybe 2x or even 4x, but not 12x
> >
> Your system A has the absolute worst case Raid 5, 3 drives. The more
> drives you add to Raid 5 the better it gets but it will never beat Raid
> 10. On top of it being the worst case, pg_xlog is not on a separate
> spindle.

True for writes, but not for reads.

> Your system B has a MUCH better config. Raid 10 is faster than Raid 5 to
> begin with but on top of that you have more drives involved plus pg_xlog
> is on a separate spindle.

I absolutely agree, it is a much better config, thats why we bought it
;)..  In system A, the xlog was actualy on the RAID 1, so it was
infact on a seperate spindle set.

> I'd say I am not surprised by your performance difference.

I'm not surprised at all that the new system outperformed the old,
it's more the factor of improvement.  12x is a _VERY_ big performance

> > Bad controler cards/configuration will seriously ruin your day.  3ware
> > escalade cards are very well supported on linux, and work excellently.
> >  Compaq smart array cards are not.  Bonnie++ benchmarks show a 9MB/sec
> > write, 29MB/sec read on the RAID 5, but a 172MB/sec write on the
> > 6xRAID 10, and 66MB/sec write on the RAID 1 on the 3ware.
> >
> What does bonnie say about the Raid 1 on the Compaq? Comparing the two
> Raid 1s is really the only valid comparison that can be made between
> these two machines.  Other than that you are comparing apples to
> snow shovels.

My main point is that you can spend $7k on a server and believe you
have a fast system.  The person who bought the original system was
under the delusion that it would make a good DB server.  For the same
$7k a different configuration can yield a vastly different performance
output.  This means that it's not quite apples to snow shovels. 
People who _believe_ they have an adequate config are often sorely
mistaken, and ask misguided questions about needed 20GB of RAM because
the system can't page to disk fast enough, when what they really need
is a good RAID 10 with a high quality controler.  A six drive RAID 10
is going to run a bit less than 20G of SSD.

Alex Turner

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