This series provides a way to efficiently backup VMs. * Backup to a single archive file * Backup 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
The file docs/backup-rfc.txt contains more details. Changes since v1: * fix spelling errors * move BackupInfo from BDS to BackupBlockJob * introduce BackupDriver to allow more than one backup format * vma: add suport to store vmstate (size is not known in advance) * add ability to store VM state Changes since v2: * BackupDriver: remove cancel_cb * use enum for BackupFormat * vma: use bdrv_open instead of bdrv_file_open * vma: fix aio, use O_DIRECT * backup one drive after another (try to avoid high load) Signed-off-by: Dietmar Maurer <diet...@proxmox.com> --- docs/backup-rfc.txt | 119 +++++++++++++++++++++++++++++++++++++++++++++++++++ 1 files changed, 119 insertions(+), 0 deletions(-) create mode 100644 docs/backup-rfc.txt diff --git a/docs/backup-rfc.txt b/docs/backup-rfc.txt new file mode 100644 index 0000000..5b4b3df --- /dev/null +++ b/docs/backup-rfc.txt @@ -0,0 +1,119 @@ +RFC: 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. + +Note: It turned out that taking a qcow2 snapshot can take a very long +time on larger files. + +=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, an 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. + + -- 1.7.2.5
