[libvirt-users] Pivot without copy

[Eric Blake]

On 05/19/2015 12:52 PM, Mathew Moon wrote:
> Hi,
> 
> Is it possible to "pivot" to a new image without doing blockcopy or
> blockpull?

No. Qemu does not support arbitrary reopening of a backing chain yet
(even with the 'change-backing-file' QMP command, that is just rewriting
contents of the qcow2 metadata, and not actually reopening the chain).
The only way to pivot an image on a live guest is via blockcommit or
blockcopy.  And one of the reasons that this is so is that pivoting only
makes sense if you can guarantee that both source and destination have
the same guest-visible contents - but without some block job that
synchronizes two images just prior to the pivot and then cleanly flushes
all pending I/O at the point of the pivot, you can't make that guarantee
for any disk image that is being actively modified by guest execution.

If you don't mind guest downtime, then you can save the guest state,
modify the save file to edit in the updated file name, then restore from
the saved state.

> I know how to use snapshots and blockpull to create a new image
> and pivot to using it live, but what I would like to do is to have a VM
> switch from using imageA.qcow2 to image2.qcow2 while running. I don't see
> why this wouldn't be possible since some of the existing libvirt tools can
> do this when they are done. I would love to see an example from a bash
> terminal as well as how it would be done using the python API.

blockpull is not the most efficient, and forces you to use a qcow2
destination; but it was indeed the first way to accomplish a pivot from
one disk to another.  But these days, with new enough libvirt and qemu,
the ideal pivot uses snapshot-create, blockcopy, and active blockcommit,
and lets you pivot to any destination format (does not have to be
qcow2).  Roughly:

$ # create a qcow2 wrapper
$ virsh snapshot-create-as $dom --no-metadata --disk-only \
  --diskspec vda,file=wrapA
$ # copy the backing file to the pivot location, ideally \
  taking advantage of faster-than-cp methods
$ $your_cool_cp imageA imageB
$ # create a destination qcow2 file
$ qemu-img create -f qcow2 -obacking_file=imageB,backing_fmt=$fmt wrapB
$ # block copy - note below that you may need to make $dom transient
$ virsh blockcopy $dom vda wrapB --shallow --reuse-external --pivot \
  --verbose
$ # commit the wrapper back into the pivoted base
$ virsh blockcommit $dom vda --shallow --active --verbose --pivot
$ # delete the temporaries
$ rm wrapA wrapB

If $your_cool_cp is fast, such as taking advantage of a copy-on-write
primitive of your storage array, then the overall operation can complete
in under a second, because the amount of data stored in the temporary
wrapA and copied over to wrapB and finally merged into imageB is so
small that it doesn't take long (much faster than blockpull, which
copies the entire disk contents regardless of how much changed in the
meantime).

qemu 2.4 will be adding persistent bitmaps, and the hope is that libvirt
can take advantage of that to allow blockcopy to work without requiring
a transient domain.  But in the meantime, you may have to use 'virsh
dumpxml $dom > $file', 'virsh undefine $dom' prior to the blockcopy,
then 'edit $file to updated name', 'virsh define $file' afterwards to
restore it to persistent.

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

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