From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tomasz Chmielewski Subject: making snapshots with raw devices? and some general snapshot thoughts Date: Wed, 25 Feb 2009 14:20:42 +0100 Message-ID: <49A545AA.60508@wpkg.org> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit To: "kvm@vger.kernel.org" Return-path: Received: from mx03.syneticon.net ([78.111.66.105]:57562 "EHLO mx03.syneticon.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753487AbZBYNUR (ORCPT ); Wed, 25 Feb 2009 08:20:17 -0500 Received: from localhost (filter1.syneticon.net [192.168.113.83]) by mx03.syneticon.net (Postfix) with ESMTP id 0DB6936149 for ; Wed, 25 Feb 2009 14:20:15 +0100 (CET) Received: from mx03.syneticon.net ([192.168.113.84]) by localhost (mx03.syneticon.net [192.168.113.83]) (amavisd-new, port 10025) with ESMTP id ZlRxH1b+nj6u for ; Wed, 25 Feb 2009 14:20:13 +0100 (CET) Received: from [192.168.10.145] (koln-5d818639.pool.einsundeins.de [93.129.134.57]) by mx03.syneticon.net (Postfix) with ESMTPSA for ; Wed, 25 Feb 2009 14:20:12 +0100 (CET) Sender: kvm-owner@vger.kernel.org List-ID: 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? [1] http://bellard.org/qemu/qemu-doc.html#SEC18 -- Tomasz Chmielewski http://wpkg.org