[PATCH] qemu: backup: Install bitmap for incremental backup to appropriate node only

[Eric Blake]

On 11/13/20 8:55 AM, Peter Krempa wrote:
> Libvirt's backup code has two modes:
> 
> 1) push - where qemu actively writes the difference since the checkpoint
>           into the output file
> 
> 2) pull - where we instruct qemu to expose a frozen disk state along
>           with a bitmap of blocks which changed since the checkpoint
> 
> For push mode qemu needs the temporary bitmap we use where we calculate
> the actual changes to be present on the block node backing the disk.
> 
> For pull mode where we expose the bitmap via NBD qemu actually wants the
> bitmap to be present for the exported block node which is the scratch
> file.
> 
> Until now we've calculated the bitmap twice and installed it both to the
> scratch file and to the disk node, but we don't need to since we know
> when it's needed.
> 
> Pass in the 'pull' flag and decide where to install the bitmap according
> to it and also when to register the bitmap name with the blockjob.
> 
> Signed-off-by: Peter Krempa <pkrempa at redhat.com>
> ---
>  src/qemu/qemu_backup.c | 42 ++++++++++++++++++++++++++----------------
>  1 file changed, 26 insertions(+), 16 deletions(-)

Reviewed-by: Eric Blake <eblake at redhat.com>


> +    /* For pull-mode backup, we need the bitmap to be present in the scratch
> +     * file as that will be exported. For push-mode backup the bitmap is
> +     * actually required on top of the image backing the disk */
> 
> -    if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
> -                                             dd->store,
> -                                             dd->incrementalBitmap,
> -                                             dd->backupdisk->incremental,
> -                                             actions,
> -                                             blockNamedNodeData) < 0)
> -        return -1;
> +    if (pull) {
> +        if (qemuBackupDiskPrepareOneBitmapsChain(dd->domdisk->src,
> +                                                 dd->store,
> +                                                 dd->incrementalBitmap,
> +                                                 dd->backupdisk->incremental,
> +                                                 actions,
> +                                                 blockNamedNodeData) < 0)

Technically, with both qemu 5.0 and qemu 5.2, the bitmap can live in the
backing store for BOTH modes, (in 5.2 and beyond, because qemu will
chase through the backing chain to find the named bitmap if it is not
present in the temporary store; in 5.0 because the temporary store is
not a filter node).  But since qemu 5.1 used a filter node for the
temporary store but was unable to chase through filter nodes, always
placing the bitmap in the temporary node for pull mode is fine.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org