Date: Sun, 11 Feb 2007 15:04:44 +0100 From: "Patrick M. Hausen" <hausen@punkt.de> To: Artem Kuchin <matrix@itlegion.ru> Cc: freebsd-stable@freebsd.org Subject: Re: What is a good choice of sata-ii raid controller for freebsd? Message-ID: <20070211140444.GB40782@hugo10.ka.punkt.de> In-Reply-To: <01e601c74c5d$31be19c0$0c00a8c0@Artem> References: <00ad01c74b65$79db1710$0c00a8c0@Artem> <20070208094620.GA9599@rink.nu> <00a701c74b6e$7c3e4550$fe03a8c0@claylaptop> <20070208165224.GA35610@icarus.home.lan> <45CC72D4.9040104@lxnt.info> <01e601c74c5d$31be19c0$0c00a8c0@Artem>
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Hello! On Fri, Feb 09, 2007 at 06:15:53PM +0300, Artem Kuchin wrote: > Under gmirror OS must issue two commands to write to disks and some > commands to check/set mark that mirrored data is intact. > Under hardware RAID OS issue sonly one command to write and no > checking command, since raid controller handles this async. > > So, software OS raid must be slower than controller based raid anyway. Yes. The OS has got to do a bit more work that is otherwise done by the CPU on the RAID controller. For modern CPUs this extra work is measurably neglegible. One guy that I happen to know, who was responsible for the database backend servers of Germany's biggest web mail provider at the time, ran extensive benchmarks. Result: for RAID 1, RAID 0 and RAID 1+0 there is no difference in "hardware RAID" vs. OS mirroring and striping. He used Linux, but I'd bet a huge amount that his findings can be transferred to arbitrary current operating systems. RAID 5 and RAID 6 are different beasts alltogether, but you do not want RAID 5 for transaction heavy systems, anyway. When you are running a huge DB that is not "read mostly", you want to have your working set in memory. If the database needs to write to disk, eventually, it's all about latency. And latency on RAID 5 is horrendous, regardless if implemented in "hardware RAID" or not. Kind regards, Patrick -- punkt.de GmbH * Vorholzstr. 25 * 76137 Karlsruhe Tel. 0721 9109 0 * Fax 0721 9109 100 info@punkt.de http://www.punkt.de Gf: Jürgen Egeling AG Mannheim 108285
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