[libvirt] Downloading and wiping assumes volume is a device or file

Eric Blake eblake at redhat.com
Thu Feb 9 20:27:05 UTC 2012

On 02/09/2012 01:04 PM, Wido den Hollander wrote:
> Hi,
> I'm still working on the RBD (RADOS / Ceph) storage driver for libvirt
> and I noticed the virStorageVolDownload and virStorageVolWipe methods.
> I assumed those would be passed on to the storage backend, but it doesn't.
> In the storageDriver the method storageVolumeDownload simply opens a
> file descriptor and reads the device.
> Until now libvirt only had support for storage drivers who presented
> regular files or block devices, but RBD doesn't. (Well, RBD could, but
> I'm currently going for Qemu-RBD).
> In the future we might see more storage drivers in libvirt for a project
> like Sheepdog as well.
> Sheepdog and RBD both have drivers in Qemu.
> What would the way be to approach this? Should the download, upload and
> wipe methods be moved to the storage backends?
> There could also be an exception? If virStoragePoolType matches
> could be invoked instead of opening the file descriptor?
> Any thoughts on this?

For at least virStorageVolDownload, I can see two useful behaviors, and
think we need to introduce a flag to specify which behavior:

virStorageVolDownload(, 0) => verbatim copy of the raw volume, including
metadata, as seen by the host; does not require any decryption keys to
download, but may require secrets for accessing remote Sheepdog serves -
great for cloning volumes

virStorageVolDownload(, CONTENTS) => copy the contents of the volume as
they would be seen in the guest; for any encrypted storage formats, this
requires the right virSecret object to access the contents - great for
converting arbitrary volumes into a raw storage format

Conversely, virStorageVolUpload(, 0) installs a raw volume, and
virStroageVolUpload(, CONTENTS) takes raw data and injects it into the
contents that the guest will see (which may involve compression or

Even on just local file systems, this would be the difference between
'cp qcow2.img1 qcow2.img2' (copy the entire metadata, without knowing
the contents as seen by the guest) and 'qemu-img conviert -f qcow2 -O
qcow2 qcow2.img1 qcow2.img2' (copy the entire guest contents, but by
creating new qcow2 metadata, which may result in a smaller img2 file as
abandoned sectors in img1 are elided).

Very much worth doing!

I also think that virStorageVolWipe should have a raw vs. contents, as in:

virStorageVolWipe(, 0) => wipe the entire volume, including any storage
metadata; a qcow2 file with 10M in use, 30M capacity, and occupying 20M
on disk gets wiped into a raw file with 20M in use, capacity, and allocation

virStorageVolWipe(, CONTENTS) => ensure that the guest sees an empty
volume, but preserve host metadata; a qcow2 file with 10M in use, 30M
capacity, and occupying 20M on disk gets converted to a qcow2 with 0
bytes in use, 30M capacity; and we probably need a second flag to
control whether disk usage is altered (that is, whether the image is
thin or pre-allocated on disk)

It certainly sounds like we should be wiring more of these decisions
through to the backends, rather than attempting to do them all from the
main storage driver.

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

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