From: "Amir G." Subject: Re: LVM vs. Ext4 snapshots (was: [PATCH v1 00/30] Ext4 snapshots) Date: Sat, 11 Jun 2011 08:41:38 +0300 Message-ID: References: <20110610101142.GA10144@ubuntu> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Lukas Czerner , Mike Snitzer , linux-ext4@vger.kernel.org, tytso@mit.edu, linux-kernel@vger.kernel.org, lvm-devel@redhat.com, linux-fsdevel To: Joe Thornber Return-path: In-Reply-To: <20110610101142.GA10144@ubuntu> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Fri, Jun 10, 2011 at 1:11 PM, Joe Thornber wro= te: > On Fri, Jun 10, 2011 at 11:01:41AM +0200, Lukas Czerner wrote: >> On Fri, 10 Jun 2011, Amir G. wrote: >> >> > CC'ing lvm-devel and fsdevel >> > >> > >> > On Wed, Jun 8, 2011 at 9:26 PM, Amir G. wrote: >> > For the sake of letting everyone understand the differences and tr= ade >> > offs between >> > LVM and ext4 snapshots, so ext4 snapshots can get a fair trial, I = need >> > to ask you >> > some questions about the implementation, which I could not figure = out by myself >> > from reading the documents. > > First up let me say that I'm not intending to support writeable > _external_ origins with multisnap. =A0This will come as a suprise to > many people, but I don't think we can resolve the dual requirements t= o > efficiently update many, many snapshots when a write occurs _and_ mak= e > those snapshots quick to delete (when you're encouraging people to > take lots of snapshots performance of delete becomes a real issue). > If I understand this article correctly: http://people.redhat.com/mpatocka/papers/shared-snapshots.pdf It says that _external_ origin write updates can be efficient to readon= ly (or not written) snapshots. Could you not support readonly snapshots of an _external_ origin? You could even support writable snapshots, that will degrade write performance to origin temporarily. It can be useful, if one wants to "try-out" mounting a temporary writable snapshot, when the origin is not even mounted. After the "try-out", the temporary snapshot can be deleted and origin write performance would go back to normal. Is that correct? Amir.