Re: [Qemu-devel] Live Block Migration using Mirroring

[Paolo Bonzini]

Il 28/02/2012 16:47, Stefan Hajnoczi ha scritto:
> At this stage we have dst/hd0snap1 opened with BDRV_O_NO_BACKING.  If
> it has no backing file and the guest issues a write request that is
> smaller than a cluster in the image file, the untouched areas of that
> cluster will be populated with zeroes.
> 
> Once dst/hd0snap1 is reopened with dst/hd0base in place there will be
> zeros in clusters where the guest wrote only a few sectors.  We will
> not "see" the backing file data in those clusters.
> 
> Have you hit this problem or did I miss something?

I'm afraid not.

Federico, perhaps you have to rewrite blkmirror to reuse copy-on-read
mechanism.  You can implement is_allocated and make the base image a
real backing_file even though you can write to it.  Perhaps some hack
with BDRV_O_NO_BACKING, or perhaps it just works with Jeff's open-on-top
behavior.

Looks like it would also make streaming of the base image Just Work, at
least to a non-raw destination.

Remaining problems:

1) differentiating writes from copy-on-read and writes from the guest.
This is needed to avoid spurious writes to the backing image each time
you're doing a copy-on-read.

2) triggering a copy-on-read before writing partial clusters.

Stefan, what do you think?

Paolo