From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: making snapshots with raw devices? and some general snapshot thoughts Date: Wed, 25 Feb 2009 09:44:33 -0600 Message-ID: <49A56761.8090209@codemonkey.ws> References: <49A545AA.60508@wpkg.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Cc: "kvm@vger.kernel.org" To: Tomasz Chmielewski Return-path: Received: from qw-out-2122.google.com ([74.125.92.26]:53488 "EHLO qw-out-2122.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753006AbZBYPpI (ORCPT ); Wed, 25 Feb 2009 10:45:08 -0500 Received: by qw-out-2122.google.com with SMTP id 5so126111qwi.37 for ; Wed, 25 Feb 2009 07:45:06 -0800 (PST) In-Reply-To: <49A545AA.60508@wpkg.org> Sender: kvm-owner@vger.kernel.org List-ID: Tomasz Chmielewski wrote: > Is it possible to make snapshots when using raw devices (i.e. disk, > partition, LVM volume) as guest's disk image? > > According to documentation[1] (and some tests I made) it is only > possible with qcow2 images. Which makes it very inflexible: > > - one is forced to use a potentially slower file access > - one can't use the benefits of i.e. iSCSI disk access, SAN etc. > > > Also, according to the documentation, "VM snapshots are snapshots of > the complete virtual machine including CPU state, RAM, device state > and the content of all the writable disks". > > Which leads to another observation: in some situations what one really > needs is to "pause" execution of guest from within the host (i.e. > because you want to shutdown or reboot the host due to a kernel or > hardware upgrade). > After all changes on host are done, "guest" should be resumed without > even knowing it was paused (most likely it will loose network > connections). > > This is how it's done with Xen or other virtualization solutions - > they only save RAM, CPU, device state to a separate file; they don't > save the content of all disks, because it's normally not needed for > "pausing" guests. > > Is it possible to do it with KVM? In monitor: (qemu) stop (qemu) migrate exec:dd of=state.img reboot machine qemu -incoming exec:dd if=state.img -other -options Regards, Anthony Liguori > > > [1] http://bellard.org/qemu/qemu-doc.html#SEC18 > >