From owner-freebsd-questions@FreeBSD.ORG Tue Jun 10 19:37:46 2008 Return-Path: Delivered-To: freebsd-questions@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 20ECA1065671 for ; Tue, 10 Jun 2008 19:37:46 +0000 (UTC) (envelope-from erikt@midgard.homeip.net) Received: from ch-smtp01.sth.basefarm.net (ch-smtp01.sth.basefarm.net [80.76.149.212]) by mx1.freebsd.org (Postfix) with ESMTP id 9B1648FC1F for ; Tue, 10 Jun 2008 19:37:45 +0000 (UTC) (envelope-from erikt@midgard.homeip.net) Received: from c83-253-25-183.bredband.comhem.se ([83.253.25.183]:60621 helo=falcon.midgard.homeip.net) by ch-smtp01.sth.basefarm.net with esmtp (Exim 4.68) (envelope-from ) id 1K69f5-0003Pf-4j for freebsd-questions@freebsd.org; Tue, 10 Jun 2008 21:37:43 +0200 Received: (qmail 78118 invoked from network); 10 Jun 2008 21:37:42 +0200 Received: from owl.midgard.homeip.net (10.1.5.7) by falcon.midgard.homeip.net with ESMTP; 10 Jun 2008 21:37:42 +0200 Received: (qmail 68366 invoked by uid 1001); 10 Jun 2008 21:37:42 +0200 Date: Tue, 10 Jun 2008 21:37:42 +0200 From: Erik Trulsson To: Wojciech Puchar Message-ID: <20080610193742.GA68256@owl.midgard.homeip.net> References: <484EACEB.7169.43FE1258@iwrtech.iwr.ru.ac.za> <20080610165435.M68290@wojtek.tensor.gdynia.pl> <20080610150536.GA67056@owl.midgard.homeip.net> <20080610171129.K75322@wojtek.tensor.gdynia.pl> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20080610171129.K75322@wojtek.tensor.gdynia.pl> User-Agent: Mutt/1.5.18 (2008-05-17) X-Originating-IP: 83.253.25.183 X-Scan-Result: No virus found in message 1K69f5-0003Pf-4j. X-Scan-Signature: ch-smtp01.sth.basefarm.net 1K69f5-0003Pf-4j 12c942c1021e2d60028976b022ba9fdf Cc: freebsd-questions@freebsd.org, DA Forsyth Subject: Re: xRAID disks.... X-BeenThere: freebsd-questions@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: User questions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Tue, 10 Jun 2008 19:37:46 -0000 On Tue, Jun 10, 2008 at 05:13:31PM +0200, Wojciech Puchar wrote: > x> hardware support. > > > >> actually there is not much need to have it for RAID-0/1/10 where there is > >> almostnothing to process. > > > > For mirrors it can actually be a big win with hardware support. > > If you use software RAID then you will have to perform each write twice > > (once to each disk), > in parallel > > while with hardware support for RAID you only need > > to transfer the data once. > > which saves at most 100MB/s bandwidth - compare this to 5-10GB/s in modern > machines. You do not normally have that much bandwidth even in a modern machine. Typical bandwidth for the northbridge/southbridge connection is 1-2 GB/s for most machines sold today. (For example just about all machines with a recent Intel desktop chipset. The connection between north- and south-bridge on those is equivalent to a PCI-E x4 connection (which provides 1GB/s in each direction.)) And that is for modern machines. Older ones have even less bandwidth available. > > If the controller resides on a PCI-bus together > > with several other devices (which is not uncommon) then the reduced > > bandwidth usage can be very useful. > > true. but not if it's builtin in chipset or on PCI express. PCI-E controller cards are still fairly uncommon, and many of them require a x4 or x8 slot, while most motherboards only have x1 slots (apart from the x16 slot intended for a graphics card.) (And PCI-express is still fairly new, so there are lots of computers in use that do not have any PCI-E slots at all.) If you go back just a few years you will find that chipset itself provides only two IDE-channels and nothing more. Any other devices reside on a single PCI-bus (which provides a total bandwidth of 133MB/s.) > > > there are really not worth price. unless you need RAID-5. > > but with todays disk prices it's better to just use RAID-1+0 and bigger > drives. That depends on what your goals are, and what constraints you operate under. RAID 10 is nice, but it requires more disks then RAID5. Extra disks create extra noise and require more power and generate more heat and (most importantly) require extra space. There are a limited amount of space available in most computer cases, which might not be able to accomodate the extra disks needed for RAID10. > > with software RAID you are not forced to operate on whole disks. usually > not everything has to be mirrored. If you have reason to use mirroring at all, then I would say that just about everything should be mirrored. (RAID is in no way a substitute for backups. The main reason for using RAID is either performance (which is often better served by several independent disks anyway) or to minimise downtime. If some parts of your disks are not mirrored then you be able to avoid that downtime anyway.) -- Erik Trulsson ertr1013@student.uu.se