Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id ; Wed, 20 Nov 2002 20:37:52 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id ; Wed, 20 Nov 2002 20:37:52 -0500 Received: from nat-pool-rdu.redhat.com ([66.187.233.200]:20284 "EHLO flossy.devel.redhat.com") by vger.kernel.org with ESMTP id ; Wed, 20 Nov 2002 20:37:50 -0500 Date: Wed, 20 Nov 2002 20:46:25 -0500 From: Doug Ledford To: Neil Brown Cc: Joel Becker , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org Subject: Re: RFC - new raid superblock layout for md driver Message-ID: <20021121014625.GA14063@redhat.com> Mail-Followup-To: Neil Brown , Joel Becker , linux-kernel@vger.kernel.org, linux-raid@vger.kernel.org References: <15835.2798.613940.614361@notabene.cse.unsw.edu.au> <20021120160259.GW806@nic1-pc.us.oracle.com> <15836.7011.785444.979392@notabene.cse.unsw.edu.au> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <15836.7011.785444.979392@notabene.cse.unsw.edu.au> User-Agent: Mutt/1.4i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2473 Lines: 46 On Thu, Nov 21, 2002 at 10:31:47AM +1100, Neil Brown wrote: > I see MD and DM as quite different, though I haven't looked much as DM > so I could be wrong. I haven't yet played with the new dm code, but if it's like I expect it to be, then I predict that in a few years, or maybe much less, md and dm will be two parts of the same whole. The purpose of md is to map from a single logical device to all the underlying physical devices. The purpose of :VM code in general is to handle the creation, orginization, and mapping of multiple physical devices into a single logical device. LVM code is usually shy on advanced mapping routines like RAID5, relying instead on underlying hardware to handle things like that while the LVM code itself just concentrates on physical volumes in the logical volume similar to how linear would do things. But, the things LVM does do that are very handy, are things like adding a new disk to a volume group and having the volume group automatically expand to fill the additional space, making it possible to increase the size of a logical volume on the fly. When you get right down to it, MD is 95% advanced mapping of physical disks with different possibilities for redundancy and performance. DM is 95% advanced handling of logical volumes including snapshot support, shrink/grow on the fly support, labelling, sharing, etc. The best of both worlds would be to make all of the MD modules be plug-ins in the DM code so that anyone creating a logical volume from a group of physical disks could pick which mapping they want used; linear, raid0, raid1, raid5, etc. You would also want all the md modules inside the DM/LVM core to support the advanced features of LVM, with the online resizing being the primary one that the md modules would need to implement and export an interface for. I would think that the snapshot support would be done at the LVM/DM level instead of in the individual md modules. Anyway, that's my take on how the two *should* go over the next year or so, who knows if that's what will actually happen. -- Doug Ledford 919-754-3700 x44233 Red Hat, Inc. 1801 Varsity Dr. Raleigh, NC 27606 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/