From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mike Snitzer Date: Tue, 5 Apr 2011 11:56:28 -0400 Subject: snapshot in shared disk environment In-Reply-To: <4D9AED98.1080900@ce.jp.nec.com> References: <4D9AED98.1080900@ce.jp.nec.com> Message-ID: <20110405155628.GC20046@redhat.com> List-Id: To: lvm-devel@redhat.com MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit On Tue, Apr 05 2011 at 6:23am -0400, Jun'ichi Nomura wrote: > Hi, > > Is it possible or is there on-going work to use snapshot in shared disk > environment? > > > In virtualization context, snapshotting is useful for quickly > creating a guest image from a template disk image. > (Fast creation and space efficiency with a cost of runtime performance) > > LVM snapshot can be used for such a purpose, but there is a problem > if we are in shared disk environment: > a) Clustered snapshot is not yet supported. > b) Origin and snapshots are activated/deactivated as a whole set. > We can't individually activate/deactivate different snapshots on > different nodes. > > a) might be ok. > A snapshot is accessed on a host at a time, not simultaneously on multiple > hosts. Also the origin is read-only. > So snapshot itself need not be cluster aware, I think. > If there are thousands of nodes sharing the disk, but only one of them > accesses the snapshot, clustered snapshot seems overkill. > > I wonder whether we could relax the restriction in b) and allow individual > activation of snapshots. > Then, we can use snapshots in shared disk environment without clustered > snapshot support? Jon Brassow has worked on exclussive activation that allows snapshots to be activated on a specific machine. Please see lvm2.git commit 1ce9f8621bce7baa815145b7de8bd3ffca14420a Mike