Date: Sun, 11 Feb 2007 15:29:44 +0100 From: Wilko Bulte <wb@freebie.xs4all.nl> To: "Patrick M. Hausen" <hausen@punkt.de> Cc: Artem Kuchin <matrix@itlegion.ru>, freebsd-stable@freebsd.org Subject: Re: What is a good choice of sata-ii raid controller for freebsd? Message-ID: <20070211142944.GA92737@freebie.xs4all.nl> In-Reply-To: <20070211140444.GB40782@hugo10.ka.punkt.de> 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> <20070211140444.GB40782@hugo10.ka.punkt.de>
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On Sun, Feb 11, 2007 at 03:04:44PM +0100, Patrick M. Hausen wrote.. > 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. For that purpose a sensibly designed battery-backup write cache works wonders. We have tons of customers running RAID5 for DBMS use. It all really depends on what your needs are as far as I/O goes whether RAID5 will do it for you or not. Do not automatically dismiss it. RAID0+1 might be faster, but comes at a substantially higher price per GB. -- Wilko Bulte wilko@FreeBSD.org
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