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Re: questions about disk configurations

From: Josh Berkus <josh(at)agliodbs(dot)com>
To: "scott(dot)marlowe" <scott(dot)marlowe(at)ihs(dot)com>
Cc: <depesz(at)depesz(dot)pl>, <pgsql-performance(at)postgresql(dot)org>
Subject: Re: questions about disk configurations
Date: 2002-12-09 23:26:36
Message-ID: 200212091526.36232.josh@agliodbs.com (view raw or flat)
Thread:
Lists: pgsql-performance
Scott,

> Actually, first I'd try one big RAID 5 and see how it runs.  THEN I'd 
> spend time mucking around with different configs if that wasn't fast 
> enough.  If you need x performance and get 10x with a RAID 5 then move on 
> to more interesting problems.

Depends on how much time you have to spend re-installing.   IMHO, RAID 5 is 
slower that straight disks for Postgres, especially with large numbers of 
writes.  This may not be true for $1000 RAID controllers, but I have yet to 
use one.

I have a box with a low-end RAID 5 controller, and it drives like a single IDE 
drive on large UPDATE queries.   Slower, somethimes.

> Not in my experience.  I'd estimate my test box with dual 18 Gig UW scsis 
> runs about 1.5 to 1.8 times faster with the two drives in a RAID1 as if 
> a single one is used.  Bonnie confirms this.  single drive can read about 
> 25 Megs a second, a pair in a RAID1 reads at about 48 Megs a second.

This is Linux software RAID?

> But as you pointed out in your reply, it's more important to look at how 
> he's gonna drive the database.  If it has to input hundreds of short 
> queries a second, that's a whole different problem than a data warehouse 
> with 500 people throwing 8 way joins at the data all day.

Definitely.

-- 
-Josh Berkus

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