From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx08.extmail.prod.ext.phx2.redhat.com [10.5.110.12]) by int-mx02.intmail.prod.int.phx2.redhat.com (8.13.8/8.13.8) with ESMTP id o05K1fZx030472 for ; Tue, 5 Jan 2010 15:01:41 -0500 Received: from esri3.esri.com (esrismtp2.esri.com [198.102.62.103]) by mx1.redhat.com (8.13.8/8.13.8) with ESMTP id o05K1PXN026442 for ; Tue, 5 Jan 2010 15:01:26 -0500 Received: from leoray.esri.com (leoray.esri.com [10.27.102.12]) by esri3.esri.com (8.11.7p3+Sun/8.11.7) with ESMTP id o05K1P612579 for ; Tue, 5 Jan 2010 12:01:25 -0800 (PST) Received: from leoray.esri.com (leoray.esri.com [127.0.0.1]) by leoray.esri.com (8.14.3/8.14.3) with ESMTP id o05K1P6f024935 for ; Tue, 5 Jan 2010 12:01:25 -0800 Received: (from ray5147@localhost) by leoray.esri.com (8.14.3/8.14.3/Submit) id o05K1P7d024934 for linux-lvm@redhat.com; Tue, 5 Jan 2010 12:01:25 -0800 Date: Tue, 5 Jan 2010 12:01:25 -0800 From: Ray Van Dolson Message-ID: <20100105200125.GA24123@esri.com> References: <4B438ECC.5000805@alteeve.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: Subject: Re: [linux-lvm] What is holding back clustered snapshotting? Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-lvm@redhat.com On Tue, Jan 05, 2010 at 11:30:01AM -0800, Stuart D. Gathman wrote: > On Tue, 5 Jan 2010, Madison Kelly wrote: > > > I know that, currently, this isn't supported. Would someone be able to > > explain or point me at a place to read up on what is holding this feature > > back? What are the difficulties? Is it just a question of time, or are there > > certain technical hurdles in the way? > > Setting up the shapshot is just a matter of locking and coordination. > However, writes to the origin or snapshot (may) require allocating a > cluster, copying the origin data, then writing the origin. All of > this coordinated with all the machines using the VG. Apart from some > cleven invention, this requires global locking on many writes. This > is just too inefficient. Writeable snapshots I guess would be a challenge. But even read only snapshots would be great as it would theoretically make backing up large, clustered filesystems simpler. > > However, you can obtain the same effect using a SAN. Have one > machine run LVM (and raid, etc), and export LVs via AoE or iSCSI. Of > course, that LVM machine now becomes a single point of failure... > > Here's an idea (someone probably already thought of this, but..), > have one machine in a cluster elected "master" for a VG, and have all > reads/writes from other machines go through the master via AoE or > iSCSI. When failure of the "master" is detected, elect another > machine to take over as master. Sort of a rotating SAN server. > Interesting idea. :) Ray