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Re: raid10 hard disk choice

From: Robert Schnabel <schnabelr(at)missouri(dot)edu>
To: pgsql-performance(at)postgresql(dot)org
Subject: Re: raid10 hard disk choice
Date: 2009-05-21 14:34:46
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Lists: pgsql-performance
Matthew Wakeling wrote:
> On Thu, 21 May 2009, Linos wrote:
>>     i have to buy a new server and in the budget i have (small) i 
>> have to select one of this two options:
>> -4 sas 146gb 15k rpm raid10.
>> -8 sas 146gb 10k rpm raid10.
> It depends what you are doing. I think in most situations, the second 
> option is better, but there may be a few situations where the reverse 
> is true.
> Basically, the first option will only be faster if you are doing lots 
> of seeking (small requests) in a single thread. As soon as you go 
> multi-threaded or are looking at sequential scans, you're better off 
> with more discs.
> Matthew
I agree.  I think you would be better off with more disks  I know from 
my own experience when I went from 8 73gb 15k drives to 16 73gb 15k 
drives I noticed a big difference in the amount of time it took to run 
my queries.  I can't give you hard numbers but most of my queries take 
hours to run so you tend to notice when they finish 20-30 minutes 
sooner.  The second option also doubles your capacity which in general 
is a good idea.  It's always easier to slightly overbuild than try and 
fix a storage problem.

Might I also suggest that you pick up at least one spare drive while 
you're at it. 

This may be somewhat of a tangent but it speaks to having a spare drive 
on hand.  A word of warning for anyone out there considering the Seagate 
1.5TB SATA drives (ST31500341AS).  (I use them for an off-site backup of 
a backup array not pg)  I'm going through a fiasco right now with these 
drives and I wish I had purchased more when I did.  I built a backup 
array with 16 of these back in October and it works great.  In October 
these drives shipped with firmware SD17.  I needed to add another 16 
drive array but the ST31500341AS drives that are currently shipping have 
a non-flashable CC1H firmware that will not work on high port count 
Adaptec cards (5XXXX) which is what I have.  It is now impossible to 
find any of these drives with firmware compatible with my controller, 
trust me I spent a couple hours on the phone with Seagate.  When I built 
the first array I bought a single spare drive.  As soon as two drives 
die I'm going to be in the position of having to either scrap all of 
them or buy a new controller that will work with the new firmware.  If I 
hadn't bought that extra drive the array would be dead as soon as one of 
the drives goes.

My point is... if you have the means, buy at least one spare while you can.


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Subject: Re: query planner uses sequencial scan instead of index scan
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Subject: Re: raid10 hard disk choice

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