From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from resqmta-ch2-10v.sys.comcast.net ([69.252.207.42]:49080 "EHLO resqmta-ch2-10v.sys.comcast.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750878AbaKVWvA (ORCPT ); Sat, 22 Nov 2014 17:51:00 -0500 Message-ID: <54711351.1040208@pobox.com> Date: Sat, 22 Nov 2014 14:50:57 -0800 From: Robert White MIME-Version: 1.0 To: Chris Murphy , Btrfs BTRFS Subject: Re: Volume/subvolume UUID uniqueness, was: BTRFS messes up snapshot LV with origin References: In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 11/22/2014 11:52 AM, Chris Murphy wrote: > I don't know how to fix this but I've convinced myself there's at > least a small problem. And not just with LVM snapshots as in the > originating thread. Yes. You also run into this problem if you use multipath hardware but haven't configured multipath support to be transparent. In any system, when you mix layers of abstraction you get problems. LVM snapshots make "the same disk blocks" appear in multiple places but don't actually present "the same disk blocks" since writes to "those blocks" will cause COW divergence. This violates the Universally Unique constraint of the UUID promise. None of these are problems with the respective systems working at their respective jobs, they are inherent problems with the fact that a servant can not serve two masters. As far as the parentage trees of subvolumes... I've been trying to get _anybody_ on here to recognize the problems already inherent in that system as implemented. Take a couple snapshots of a subvolume, and then send those subvolumes to another file system with send/receive, and then do "btrfs subvolume list -u -q" on the two filesystems and tell me that mess makes sense. Or try to recreate a subvolume from its snapshot in a way that doesn't shatter the relationships in your backup scheme. (I'm researching for a couple patches but I'm not expecting a warm reception given the silence to date). So yea, UUID plus copy plus mirror plus snapshot equals a trip to caveat city.