Re: How to improve db performance with $7K?

Don't you think "optimal stripe width" would be
a good question to research the binaries for? I'd
think that drives the answer, largely.  (uh oh, pun alert)

EG, oracle issues IO requests (this may have changed _just_
recently) in 64KB chunks, regardless of what you ask for.
So when I did my striping (many moons ago, when the Earth
was young...) I did it in 128KB widths, and set the oracle
"multiblock read count" according. For oracle, any stripe size
under 64KB=stupid, anything much over 128K/258K=wasteful.

I am eager to find out how PG handles all this.


- Ross



p.s. <Brooklyn thug accent> 'You want a database record? I
      gotcher record right here' http://en.wikipedia.org/wiki/Akashic_Records



-----Original Message-----
From: pgsql-performance-owner@postgresql.org [mailto:pgsql-performance-owner@postgresql.org] On Behalf Of Alex Turner
Sent: Monday, April 18, 2005 2:21 PM
To: Jacques Caron
Cc: Greg Stark; William Yu; pgsql-performance@postgresql.org
Subject: Re: [PERFORM] How to improve db performance with $7K?


So I wonder if one could take this stripe size thing further and say that a larger stripe size is more likely to result
inrequests getting served parallized across disks which would lead to increased performance? 

Again, thanks to all people on this list, I know that I have learnt a _hell_ of alot since subscribing.

Alex Turner
netEconomist

On 4/18/05, Alex Turner <armtuk@gmail.com> wrote:
> Ok - well - I am partially wrong...
>
> If you're stripe size is 64Kb, and you are reading 256k worth of data,
> it will be spread across four drives, so you will need to read from
> four devices to get your 256k of data (RAID 0 or 5 or 10), but if you
> are only reading 64kb of data, I guess you would only need to read
> from one disk.
>
> So my assertion that adding more drives doesn't help is pretty
> wrong... particularly with OLTP because it's always dealing with
> blocks that are smaller that the stripe size.
>
> Alex Turner
> netEconomist
>
> On 4/18/05, Jacques Caron <jc@directinfos.com> wrote:
> > Hi,
> >
> > At 18:56 18/04/2005, Alex Turner wrote:
> > >All drives are required to fill every request in all RAID levels
> >
> > No, this is definitely wrong. In many cases, most drives don't
> > actually have the data requested, how could they handle the request?
> >
> > When reading one random sector, only *one* drive out of N is ever
> > used to service any given request, be it RAID 0, 1, 0+1, 1+0 or 5.
> >
> > When writing:
> > - in RAID 0, 1 drive
> > - in RAID 1, RAID 0+1 or 1+0, 2 drives
> > - in RAID 5, you need to read on all drives and write on 2.
> >
> > Otherwise, what would be the point of RAID 0, 0+1 or 1+0?
> >
> > Jacques.
> >
> >
>

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