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Re: Best use of second controller with faster disks?

From: Francisco Reyes <lists(at)stringsutils(dot)com>
To: Vivek Khera <vivek(at)khera(dot)org>
Cc: Pgsql performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: Best use of second controller with faster disks?
Date: 2007-06-13 00:33:03
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Lists: pgsql-performance
Vivek Khera writes:

> what raid card have you got?

2 3ware cards.
I believe both are 9550SX 
> i'm playing with an external enclosure  
> which has an areca sata raid in it and connects to the host via fibre  
> channel. 

What is the OS? FreeBSD?
One of the reasons I stick with 3ware is that it is well supported in 
FreeBSD and has a pretty decent management program

> it is wicked fast, and supports a RAID6 which seems to be  
> as fast as the RAID10 in my initial testing on this unit.

My next "large" machine I am also leaning towards RAID6. The space different 
is just too big to ignore.
3ware recommends RAID6 for 5+ drives.
> What drives are you booting from?

Booting from the 8 drive raid.

>  If you're booting from the 4-drive  
> RAID10, perhaps split that into a pair of RAID1's and boot from one  
> and use the other as the pg log disk.

Maybe for the next machine.

> however, I must say that with my 16 disk array, peeling the log off  
> the main volume actually slowed it down a bit.  I think that the raid  
> card is just so fast at doing the RAID6 computations and having the  
> striping is a big gain over the dedicated RAID1 for the log.

Could be.
Seems like RAID6 is supposed to be a good balance between performance and 
available space.

> Right now I'm testing an 8-disk RAID6 configuration on the same   
> device; it seems slower than the 16-disk RAID6, but I haven't yet  
> tried 8-disk RAID10 with dedicated log yet.

Is all this within the same controller?
> i'd bump checkpoint_segements up to 256 given the amount of disk  
> you've got dedicated to it.  be sure to increase checkpoint timeout too.

Thanks. Will try that.

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