Re: [Qemu-devel] [PATCH v5 1/6] add documenation for new backup framework

[Markus Armbruster]

Starting the series with a problem statement is a good idea.

Suggest "Subject: <subsystem>: Design documentation", where <subsystem>
is how you'd like to call the beast.  Also tack it to your other patch
subjects, except when they really patch other subsystems (use your
judgement).

More inline.

Dietmar Maurer <address@hidden> writes:

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

Dangling reference until PATCH 4/6 adds the file.  Tolerable.

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

Trailing blank lines.  Please trim if you need to respin anyway.