[libvirt] [re-send][PATCH 1/3] add qemu driver hook

[li guang]

在 2012-10-10三的 15:00 +0100，Daniel P. Berrange写道：
> On Mon, Oct 08, 2012 at 08:05:21PM -0600, Eric Blake wrote:
> > On 10/08/2012 07:51 PM, liguang wrote:
> > > this hook aimed at migration only, it supposed
> > > to be used as a helper of migrate --copy-storage-*
> > > features to remove the unreasonable limitation
> > > of pre-exist disk images at migration target.
> > > someone can add more functions to hook files.
> > 
> > NACK.  Instead of writing a hook that runs outside of libvirt and has to
> > be installed to be of use, we should instead fix libvirt to do the file
> > creation itself.  That is, you should be patching
> > src/qemu/qemu_migration.c to detect when copy-storage-* has been passed,
> > and to then pre-create the proper empty files and give them correct
> > SELinux labels.
> 
> Correctly doing auto-creation of disk images on the target host is
> alot more complicated that you might assume, and the hook script
> here misses alot of the hard bits
> 
>  - If the migration fails, you need to clean up these disk images
>    you just created, because the data in them may be incomplete
>    or corrupt.
> 
>  - If the source image is using encryption, then the target image
>    should use encryption too
> 
>  - If the source image uses a backing file, then when incremental
>    copy is requested, we might need to maintain the backing store.
> 
>  - if the migration API did not have either of the --copy-*
>    flags set we should not be doing any creation of disk images
> 
>  - We need to handling of non-file based disk images, because
>    even block devices may be local-only.
> 
>  - If the user passed a custom XML for the target that differs
>    from the source, this may affect decisions we need to make
>    above wrt encryption or backing files
> 
> Finally, we should *not* do auto-creation by default - we need to
> add a new migrate flag to allow apps to turn it on, only if they
> want it.
> 
> Daniel

perfect consideration!
suppose this situation,
I create disk images qemu needed at target by
qemu-img create -f 'format' 'image' 'size',
then virsh migrate --copy-storage-*
everything runs smoothly,
so, as i stated before, this patch limited to
create-image-by-hand alleviation.

-- 
liguang    lig.fnst at cn.fujitsu.com
FNST linux kernel team