Message-ID: <43DF9D93.3030602@drule.org>
Date: Tue, 31 Jan 2006 10:25:39 -0700
From: Kevin <kev@drule.org>
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To: pgsql-performance@postgresql.org
Subject: Re: Huge Data sets, simple queries
References: <C004D7AE.1B5A7%llonergan@greenplum.com>
In-Reply-To: <C004D7AE.1B5A7%llonergan@greenplum.com>
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Luke Lonergan wrote:
> Jim,
>
> On 1/30/06 12:25 PM, "Jim C. Nasby" <jnasby@pervasive.com> wrote:
>
>   
>> Why divide by 2? A good raid controller should be able to send read
>> requests to both drives out of the mirrored set to fully utilize the
>> bandwidth. Of course, that probably won't come into play unless the OS
>> decides that it's going to read-ahead fairly large chunks of the table
>> at a time...
>>     
>
> I've not seen one that does, nor would it work in the general case IMO.  In
> RAID1 writes are duplicated and reads come from one of the copies.  You
> could alternate read service requests to minimize rotational latency, but
> you can't improve bandwidth.
>
> - Luke   
>
>   
For Solaris's software raid, the default settings for raid-1 sets is:  
round-robin read, parallel write.   I assumed this mean't it would give 
similar read performance to raid-0, but I've never benchmarked it.

-Kevin