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Date: Mon, 31 Mar 2003 09:35:54 -0500
From: Jason Andresen <jandrese@mitre.org>
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To: Mattias Pantzare <pantzer@ludd.luth.se>
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cc: Lukas Ertl <l.ertl@univie.ac.at>
Subject: Re: vinum performance
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Mattias Pantzare wrote:
> Lukas Ertl wrote:
> 
>> Ok. But I still don't understand why RAID 5 write performance is _so_ 
>> bad.
>> The CPU is not the bottle neck, it's rather bored. And I don't understand
>> why RAID 0 doesn't give a big boost at all. Is the ahc driver known to be
>> slow?
> 
> 
> To do a RAID 5 write you do this:
> 1. Read the old data on the blocks that you will write to.
> 2. Read the coresponding parity data.
> 3. Write the new data.
> 4. Write the new parity.

Hmm, how about the case where you're writing new data?  You shouldn't 
have to do steps 1 & 2, and yet the RAID5 write performance is still 
abysmial.

I get 4565 K/sec on modern ATA/133 HDDs.

Reading is much better at 91908 K/sec at least.

-- 
   \  |_ _|__ __|_ \ __| Jason Andresen        jandrese@mitre.org
  |\/ |  |    |    / _|  Network and Distributed Systems Engineer
_|  _|___|  _| _|_\___| Office: 703-883-7755