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Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs

From: Steve Crawford <scrawford(at)pinpointresearch(dot)com>
To: "Rajesh Kumar Mallah(dot)" <mallah(at)trade-india(dot)com>,pgsql-performance(at)postgresql(dot)org
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on faster HDDs
Date: 2002-11-21 18:56:29
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Lists: pgsql-adminpgsql-performance
I had long labored under the impression that RAID 5 should give me better 
performance but I have since encountered many reports that this is not the 
case. Do some searching on Google and you will probably find numerous 

Note 3x18 w/RAID5 will give 36GB usable while 2x36 w/o RAID is 72GB. 
You could use mirroring on the 2x36 and have the same usable space.

A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a 
single disk) and an improvement on reads due to having two drives to read 
from and will (based on the Scientific Wild Ass Guess method and knowing 
nothing about your overall system) probably be faster than the RAID5 
configuration while giving you identical usable space and data safety.

You also may see improvements due to the 15,000RPM drives (of course RPM is 
sort of an arbitrary measure - you really want to know about track access 
times, latency, transfer rate, etc. and RPM is just one influencing factor 
for the above).

The quality of your RAID cards will also be important (how fast do they 
perform their calculations, how much buffer do they have) as will the overall 
specs of you system. If you have a bottleneck somewhere other than your raw 
disk I/O then you can throw all the money you want at faster drives and see 
no improvement.


On Thursday 21 November 2002 8:45 am, you wrote:
> Hi folks,
> I have two options:
> 3*18 GB 10,000 RPM Ultra160 Dual Channel SCSI  controller + H/W Raid 5
> and
> 2*36 GB 15,000 RPM Ultra320 Dual Channel SCSI and no RAID
> Does anyone opinions *performance wise*  the pros and cons of above
> two options.
> please take in consideration in latter case its higher RPM and better
> SCSI interface.
> Regds
> Mallah.

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