From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from plane.gmane.org ([80.91.229.3]:33754 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753115AbaEQMvI (ORCPT ); Sat, 17 May 2014 08:51:08 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1Wle4v-0004Ca-Ok for linux-btrfs@vger.kernel.org; Sat, 17 May 2014 14:51:05 +0200 Received: from cpc21-stap10-2-0-cust974.12-2.cable.virginm.net ([86.0.163.207]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Sat, 17 May 2014 14:51:05 +0200 Received: from m_btrfs by cpc21-stap10-2-0-cust974.12-2.cable.virginm.net with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Sat, 17 May 2014 14:51:05 +0200 To: linux-btrfs@vger.kernel.org From: Martin Subject: Re: ditto blocks on ZFS Date: Sat, 17 May 2014 13:50:52 +0100 Message-ID: References: <2308735.51F3c4eZQ7@xev> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 In-Reply-To: <2308735.51F3c4eZQ7@xev> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 16/05/14 04:07, Russell Coker wrote: > https://blogs.oracle.com/bill/entry/ditto_blocks_the_amazing_tape > > Probably most of you already know about this, but for those of you who haven't > the above describes ZFS "ditto blocks" which is a good feature we need on > BTRFS. The briefest summary is that on top of the RAID redundancy there... [... are additional copies of metadata ...] Is that idea not already implemented in effect in btrfs with the way that the superblocks are replicated multiple times, ever more times, for ever more huge storage devices? The one exception is for SSDs whereby there is the excuse that you cannot know whether your data is usefully replicated across different erase blocks on a single device, and SSDs are not 'that big' anyhow. So... Your idea of replicating metadata multiple times in proportion to assumed 'importance' or 'extent of impact if lost' is an interesting approach. However, is that appropriate and useful considering the real world failure mechanisms that are to be guarded against? Do you see or measure any real advantage? Regards, Martin