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Re: SAN performance mystery

From: Jim Nasby <jnasby(at)pervasive(dot)com>
To: Greg Stark <gsstark(at)mit(dot)edu>
Cc: "Alex Turner" <armtuk(at)gmail(dot)com>, "Mark Lewis" <mark(dot)lewis(at)mir3(dot)com>, "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "Brian Hurt" <bhurt(at)janestcapital(dot)com>, "Tim Allen" <tim(at)proximity(dot)com(dot)au>, pgsql-performance(at)postgresql(dot)org
Subject: Re: SAN performance mystery
Date: 2006-06-17 19:53:03
Message-ID: C5B2B8FE-15D8-49A7-BF44-DE32CC5D07E8@pervasive.com (view raw or flat)
Thread:
Lists: pgsql-performance
On Jun 16, 2006, at 6:28 AM, Greg Stark wrote:
> I never understood why disk caches on the order of megabytes are  
> exciting. Why
> should disk manufacturers be any better about cache management than OS
> authors?
>
> In the case of RAID 5 this could actually work against you since  
> the RAID
> controller can _only_ use its cache to find parity blocks when  
> writing.
> Software raid can use all of the OS's disk cache to that end.

IIRC some of the Bizgres folks have found better performance with  
software raid for just that reason. The big advantage HW raid has is  
that you can do a battery-backed cache, something you'll never be  
able to duplicate in a general-purpose computer (sure, you could  
battery-back the DRAM if you really wanted to, but if the kernel  
crashed you'd be completely screwed, which isn't the case with a  
battery-backed RAID controller).

The quality of the RAID controller also makes a huge difference.
--
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461




--
Jim C. Nasby, Sr. Engineering Consultant      jnasby(at)pervasive(dot)com
Pervasive Software      http://pervasive.com    work: 512-231-6117
vcard: http://jim.nasby.net/pervasive.vcf       cell: 512-569-9461



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