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Re: questions about disk configurations

From: Ron Johnson <ron(dot)l(dot)johnson(at)cox(dot)net>
To: PgSQL Performance ML <pgsql-performance(at)postgresql(dot)org>
Subject: Re: questions about disk configurations
Date: 2002-12-09 20:46:40
Message-ID: 1039466800.27242.51.camel@haggis (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Mon, 2002-12-09 at 07:05, Hubert depesz Lubaczewski wrote:
> On Mon, Dec 09, 2002 at 01:01:58PM +0100, Adrian 'Dagurashibanipal' von Bidder wrote:
> > A bit more data is needed before anybody can give you more help:
> >  - what is your budget?
> >  - how big will your databases be?
> >  - what's the read/write ratio?
> my question as for now is purely theoretical. i'm not asking about any
> specific situation, but me may talk about medium sized web size. budget
> is irrelevant (i'd like to talk *only* about harddrives, not memory,
> architescure and so on).

What is "medium sized web"?  The *system* *is* important!!  Stuffing
your box with RAM may, in fact, override your disks, if the RAM caches

> >  - as you correctly said: distribute the load on many spindles. On a
> > busy database, 4*20G is probably faster than 1*80G
> as i said: i know that 3 disks are bettar than 1 (as for postgres
> installation, because system data and swap should be on 4th disc - but
> this is obvious).
> > beyound this, experiences vary. RAID1 and RAID5 are rated differently by
> > different people - and especially with RAID5 there are (I think) really
> > performance differencies between the various products. RAID0 is fastest,
> > of course, but you probably care for your data.
> that's exactly what i'm asking about: which raid is best suited for
> which data amongst out 3 sets (xlog, tables, indices). or maybe for some
> types of data single disc is better than raid for some strange reason?
> is it better to (when having 2 discs) setup raid 0/1 or to use tham
> separatelly as xlog/tables?

These are *GENERALITIES*!!!!  _All_ is dependent on which SCSI
controller you choose, and how much cache it has!!!!!!!!

- RAID0 does *great* at both reading and writing, but everyone knows
that it is insecure.
- RAID1 does better than JBOD at reading and writing, but not as
good as RAID0.
- RAID01 and RAID10 do just about as good as RAID0.
- RAID5 does great with reads, but bad with writes, *unless* the
controller has *lots* of cache.  Then, write speeds are great.

Slightly off topic: if I have Important Data, then I would not trust
a caching controlller unless it has a battery backup.  Unfortunately,
the only "caching controlllers with battery backup" that I've seen
are pretty expensive...

| Ron Johnson, Jr.     mailto:ron(dot)l(dot)johnson(at)cox(dot)net          |
| Jefferson, LA  USA  |
|                                                            |
| "they love our milk and honey, but preach about another    |
|  way of living"                                            |
|    Merle Haggard, "The Fighting Side Of Me"                |

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