From patchwork Wed Feb 20 09:31:58 2013 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Dietmar Maurer X-Patchwork-Id: 222060 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@bilbo.ozlabs.org Received: from lists.gnu.org (lists.gnu.org [208.118.235.17]) (using TLSv1 with cipher AES256-SHA (256/256 bits)) (Client did not present a certificate) by ozlabs.org (Postfix) with ESMTPS id A2EF72C007C for ; Wed, 20 Feb 2013 22:31:42 +1100 (EST) Received: from localhost ([::1]:41401 helo=lists.gnu.org) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1U863I-0006vP-A1 for incoming@patchwork.ozlabs.org; Wed, 20 Feb 2013 04:33:24 -0500 Received: from eggs.gnu.org ([208.118.235.92]:48243) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1U862B-0004DL-6c for qemu-devel@nongnu.org; Wed, 20 Feb 2013 04:32:24 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1U8624-0002Xt-Kd for qemu-devel@nongnu.org; Wed, 20 Feb 2013 04:32:15 -0500 Received: from www.maurer-it.com ([213.129.239.114]:48634 helo=maui.maurer-it.com) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1U8624-0002XG-86 for qemu-devel@nongnu.org; Wed, 20 Feb 2013 04:32:08 -0500 Received: by maui.maurer-it.com (Postfix, from userid 0) id 0D98E3B0855; Wed, 20 Feb 2013 10:32:07 +0100 (CET) From: Dietmar Maurer To: qemu-devel@nongnu.org Date: Wed, 20 Feb 2013 10:31:58 +0100 Message-Id: <1361352723-218544-2-git-send-email-dietmar@proxmox.com> X-Mailer: git-send-email 1.7.2.5 In-Reply-To: <1361352723-218544-1-git-send-email-dietmar@proxmox.com> References: <1361352723-218544-1-git-send-email-dietmar@proxmox.com> X-detected-operating-system: by eggs.gnu.org: GNU/Linux 3.x X-Received-From: 213.129.239.114 Cc: Dietmar Maurer Subject: [Qemu-devel] [PATCH v4 1/6] add documenation for new backup framework X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org Signed-off-by: Dietmar Maurer --- docs/backup.txt | 116 +++++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 116 insertions(+), 0 deletions(-) create mode 100644 docs/backup.txt diff --git a/docs/backup.txt b/docs/backup.txt new file mode 100644 index 0000000..927d787 --- /dev/null +++ b/docs/backup.txt @@ -0,0 +1,116 @@ +Efficient VM backup for qemu + +=Requirements= + +* Backup to a single archive file +* Backup needs to contain all data to restore VM (full backup) +* Do not depend on storage type or image format +* Avoid use of temporary storage +* store sparse images efficiently + +=Introduction= + +Most VM backup solutions use some kind of snapshot to get a consistent +VM view at a specific point in time. For example, we previously used +LVM to create a snapshot of all used VM images, which are then copied +into a tar file. + +That basically means that any data written during backup involve +considerable overhead. For LVM we get the following steps: + +1.) read original data (VM write) +2.) write original data into snapshot (VM write) +3.) write new data (VM write) +4.) read data from snapshot (backup) +5.) write data from snapshot into tar file (backup) + +Another approach to backup VM images is to create a new qcow2 image +which use the old image as base. During backup, writes are redirected +to the new image, so the old image represents a 'snapshot'. After +backup, data need to be copied back from new image into the old +one (commit). So a simple write during backup triggers the following +steps: + +1.) write new data to new image (VM write) +2.) read data from old image (backup) +3.) write data from old image into tar file (backup) + +4.) read data from new image (commit) +5.) write data to old image (commit) + +This is in fact the same overhead as before. Other tools like qemu +livebackup produces similar overhead (2 reads, 3 writes). + +Some storage types/formats supports internal snapshots using some kind +of reference counting (rados, sheepdog, dm-thin, qcow2). It would be possible +to use that for backups, but for now we want to be storage-independent. + +=Make it more efficient= + +The be more efficient, we simply need to avoid unnecessary steps. The +following steps are always required: + +1.) read old data before it gets overwritten +2.) write that data into the backup archive +3.) write new data (VM write) + +As you can see, this involves only one read, and two writes. + +To make that work, our backup archive need to be able to store image +data 'out of order'. It is important to notice that this will not work +with traditional archive formats like tar. + +During backup we simply intercept writes, then read existing data and +store that directly into the archive. After that we can continue the +write. + +==Advantages== + +* very good performance (1 read, 2 writes) +* works on any storage type and image format. +* avoid usage of temporary storage +* we can define a new and simple archive format, which is able to + store sparse files efficiently. + +Note: Storing sparse files is a mess with existing archive +formats. For example, tar requires information about holes at the +beginning of the archive. + +==Disadvantages== + +* we need to define a new archive format + +Note: Most existing archive formats are optimized to store small files +including file attributes. We simply do not need that for VM archives. + +* archive contains data 'out of order' + +If you want to access image data in sequential order, you need to +re-order archive data. It would be possible to to that on the fly, +using temporary files. + +Fortunately, a normal restore/extract works perfectly with 'out of +order' data, because the target files are seekable. + +* slow backup storage can slow down VM during backup + +It is important to note that we only do sequential writes to the +backup storage. Furthermore one can compress the backup stream. IMHO, +it is better to slow down the VM a bit. All other solutions creates +large amounts of temporary data during backup. + +=Archive format requirements= + +The basic requirement for such new format is that we can store image +date 'out of order'. It is also very likely that we have less than 256 +drives/images per VM, and we want to be able to store VM configuration +files. + +We have defined a very simply format with those properties, see: + +docs/specs/vma_spec.txt + +Please let us know if you know an existing format which provides the +same functionality. + +