[libvirt-users] Looks like blockpull does not accept a subset of the entire chain of backing files

[Eric Blake]

On 01/29/2014 07:51 AM, Richard Gomes wrote:
> Hello
> 
> If I'm not terribly mistaken, looks like libvirt 1.2.1 does not provide
> ability of merging only a subset of the entire chain of backing files.

Correct - qemu doesn't provide full functionality for merging in all
directions.  And even though upstream qemu has been adding more pieces
(for example, they recently added the ability to commit the active
image), libvirt still has to be taught to exercise that.

> 
> So, if I have a chain like this:
> 
>                  root <- a <-b <- c <- d <- active
> 
> ... and  I'd like to obtain a chain like this:
> 
>                   root <-  c <- d <- active

Are you trying to squash a and b into root (commit direction, in which
case, the existing block-commit command works just fine for this
scenario) or into c (pull direction, in which case, qemu doesn't support
it yet)?

> The point is: How could I obtain the results I'm trying to achieve via
> command (1) ?

In the current blockpull implementation, qemu refuses to pull into
anything but the active image (true even for your use of qemu 1.7).
I've been asking for the ability to pull into intermediate images, but
it isn't there yet.

> 
> I'm new to libvirt, but the article below made me think that what I'm
> trying to do would be possible:

That said, it IS possible to fake the same effect, using a series of
block-rebase and raw qemu-img commands (not heavily tested, so you may
still have to tweak the actions you use).  Also, block mirroring doesn't
yet work with persistent disks (because qemu doesn't yet have a way to
resume an operation across restarts), so you temporarily have to make
your domain transient (virsh undefine $dom); you can redefine it at the
end of the sequence.

Start from root <- a <- b <- c <- d <- active

Create c1 with 'cp c c1' for this tree:
root <- a <- b \- c <- d <- active
                - c1

Merge a and b into c1 with 'qemu-img rebase -b base c1' for this tree:
root \- a <- b <- c <- d <- active
      ------------c1

Create d1 with 'cp d d1' for this tree:
root \- a <- b <- c \- d <- active
      ------------c1 - d1

Use 'qemu-img rebase -u -b c1 d1' for this tree:
root \- a <- b <- c <- d <- active
      ------------c1 <- d1

Create a blank destination active file wrapping d1, with 'qemu-img
create -f qcow2 -o backing_file=d1,backing_fmt=qcow2 active1' for this tree:
root \- a <- b <- c <- d <- active
      ------------c1 <- d1 <- active1

Set up a block mirroring, and pivot over to the new chain once it is
stable, with 'virsh blockcopy $dom active --dest active1 --shallow
--reuse-external --wait --pivot', for this tree:
root \------------c1 <- d1 <- active1
      - a <- b <- c <- d <- active

Clean up - a, b, c, d, and active are now no longer in use, and both
qemu and libvirt see the shorter chain (albeit with new file names),
with virtually no guest downtime.

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

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