From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeff Garzik Subject: Re: Notes on support for multiple devices for a single filesystem Date: Wed, 17 Dec 2008 16:30:00 -0500 Message-ID: <49496F58.4030003@garzik.org> References: <1227183484.6161.17.camel@think.oraclecorp.com> <1228962896.21376.11.camel@think.oraclecorp.com> <20081211141436.030c2d65.sfr@canb.auug.org.au> <20081210200604.8e190b0d.akpm@linux-foundation.org> <1229006596.22236.46.camel@think.oraclecorp.com> <20081215210323.GB5000@webber.adilger.int> <20081217132343.GA14695@infradead.org> <20081217115325.3312858a.akpm@linux-foundation.org> <1229547485.27170.77.camel@think.oraclecorp.com> <20081217212409.GB5000@webber.adilger.int> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Chris Mason , Andrew Morton , Christoph Hellwig , sfr@canb.auug.org.au, linux-kernel@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-fsdevel@vger.kernel.org To: Andreas Dilger Return-path: Received: from srv5.dvmed.net ([207.36.208.214]:38946 "EHLO mail.dvmed.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751742AbYLQVaO (ORCPT ); Wed, 17 Dec 2008 16:30:14 -0500 In-Reply-To: <20081217212409.GB5000@webber.adilger.int> Sender: linux-fsdevel-owner@vger.kernel.org List-ID: Andreas Dilger wrote: > I can't speak for btrfs, but I don't think multiple device access from > the filesystem is a "layering violation" as some people comment. It is > just a different type of layering. With ZFS there is a distinct layer > that is handling the allocation, redundancy, and transactions (SPA, DMU) > that is exporting an object interface, and the filesystem (ZPL, or future > versions of Lustre) is built on top of that object interface. Furthermore... think about object-based storage filesystems. They will need to directly issue SCSI commands to storage devices. Call it a layering violation if you will, but you simply cannot even pretend that an OSD is a linear block device for the purposes of our existing block layer. Jeff