From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S263598AbTJQTY1 (ORCPT ); Fri, 17 Oct 2003 15:24:27 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S263599AbTJQTY1 (ORCPT ); Fri, 17 Oct 2003 15:24:27 -0400 Received: from unthought.net ([212.97.129.88]:7598 "EHLO unthought.net") by vger.kernel.org with ESMTP id S263598AbTJQTYV (ORCPT ); Fri, 17 Oct 2003 15:24:21 -0400 Date: Fri, 17 Oct 2003 21:24:19 +0200 From: Jakob Oestergaard To: =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= Cc: linux-kernel@vger.kernel.org Subject: Re: Software RAID5 with 2.6.0-test Message-ID: <20031017192419.GG8711@unthought.net> Mail-Followup-To: Jakob Oestergaard , =?iso-8859-1?Q?M=E5ns_Rullg=E5rd?= , linux-kernel@vger.kernel.org References: <1065690658.10389.19.camel@slurv> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: Mutt/1.3.28i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Oct 17, 2003 at 07:44:31PM +0200, Måns Rullgård wrote: ... > > What about the RAID controllers in the $400 category? Surely, they > must be doing something better than the $50 fakeraid controllers. The RAID controllers in the $400 category are exactly the ones who include (inferior) processors, and add an extra layer in the 'chain of command'. The inexpensive 'fakeraid' controllers include some form of SW RAID in their driver, and could therefore perform as well as Linux SW RAID (if the driver's RAID code is as clever and efficient as the Linux SW RAID code - which it may not be). Ironic, maybe ;) I have never tried using one of the 'fakeraid' controllers, so I can't speak for their actual real-world performance. They could include a crap IDE controller, or they could have crap drivers - both could give poor performance. Could, would, should, don't know... However, the "hardware RAID" controllers that were discussed here would be the ones in the $400->$(obscene) price range. Now that I'm posting anyway - I thought of a plus for the HW RAID controllers (hey, they're way behind on the scoreboard so far, so I might as well be a gentleman and give them a point or two): *) Battery backed write cache This will allow the controller to say 'ok I'm done with your sync()', way before the data actually reaches the disk platters. For some workloads this can be a big win. For some odd reason, it seems that 'most' (read: all that I could find) HW RAID controllers are unable to control more than 256 MB of memory - which is odd. That amound of memory is almost so low that it defeats the purpose of having it in the first place. But again, some workloads may well benefit. And this is something you just can't do with SW RAID (at least not before someone implements support for an NVRAM buffer store in the Linux I/O layer). While on that topic: SDRAM PCI cards with battery backup can be had relatively cheap up to at least a few gigabytes. It'd be pretty cool to have the kernel recognize that for buffer storage. I could see the fun in doing half a million random writes and a sync(), and having the system tell me it's done the microsecond I issue the sync(). Enough with the daydreaming already ;) -- ................................................................ : jakob@unthought.net : And I see the elder races, : :.........................: putrid forms of man : : Jakob Østergaard : See him rise and claim the earth, : : OZ9ABN : his downfall is at hand. : :.........................:............{Konkhra}...............: