Re: [Qemu-devel] [PATCH v3 1/6] RFC: Efficient VM backup for qemu

[Eric Blake]

On 02/19/2013 04:31 AM, Dietmar Maurer wrote:
> 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

It is customary to send a 0/6 cover letter for details like this, rather
than slamming it into the first patch (git send-email --cover-letter).
Remember, once it is in git, it is no longer as easy to identify where a
series starts and ends, so the contents of the cover letter is not
essential to git history, just to reviewers.

> 
> The file docs/backup-rfc.txt contains more details.

While naming the file *-rfc is fine for an RFC patch series, it better
not be the final name that you actually want committed.

> 
> 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)

Also, it is customary to list series revision history after the ---
separator; again, something useful for reviewers, but pointless in the
actual git history.

> 
> Signed-off-by: Dietmar Maurer <address@hidden>
> ---
>  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

You already have RFC in the subject line; you don't need it here in your
proposed contents.

> +
> +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)

Shouldn't that be '(VM read)'?

> +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.

That's an independent issue, and there have been patches proposed to try
and reduce that time.

> +
> +=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.

s/an/and/

> +
> +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.

Are you also requiring that the output file descriptor be seekable?  Tar
has the advantage of using a pipe; requiring a seekable file might be an
acceptable tradeoff, but it does limit what you can do when you can't
pass a pipe in for the destination.

> +
> +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

This file should be part of the same patch that first mentions it.

> +
> +Please let us know if you know an existing format which provides the
> +same functionality.
> +
> +
> 

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

signature.asc
Description: OpenPGP digital signature