On 05/14/2013 03:16 AM, Wido den Hollander wrote:
Hi,
While working on the CloudStack code for 4.2 I'm running into some
problems with QCOW2 and RBD format 2.
A quick cap of the problem:
CloudStack stores all the templates on "Secondary Storage" as QCOW2 files.
When a VM is deployed, this image will be copied (once) from Secondary
Storage to Primary Storage, this is done by qemu-img which does the
conversion.
When the image is on Primary Storage it will be copied to the
destination volume and the Instance/VM is started.
In that last step I want to use layering, but the problem is that my
image is RBD format 1 due to the qemu-img copy.
I can skip the qemu-img copy, but I have to find a way to go from QCOW2
to RBD format 2 in preferably one step. There is a issue about this in
the tracker [0], but that won't be finished on time.
Another idea would be to do QCOW2 -> RAW -> RBD format 2, but that extra
step will take a lot of time. It would also require some temporary
storage on the hypervisor.
I also thought of creating the RBD image prior to converting to it, but
that fails, since qemu-img won't let you convert to a already existing
image.
Any brights ideas? Maybe even doing the QCOW2 -> RAW format in memory?
In Java?
One way that wouldn't require a change in qemu would be adding ceph
configuration options to change the behavior of librbd's rbd_create[2|3]().
For example, there could be just:
rbd_create_default_format UINT default 1
or we could add other options to adjust the rest of the image creation
parameters i.e.:
rbd_create_default_object_order INT default 22
// these only apply to format 2
rbd_create_default_features UINT64 default 1 # 1 for layering, 3 for
layering+stripingv2
rbd_create_default_stripe_count UINT64 default 1 # changing requires
stripingv2 feature
rbd_create_default_stripe_unit UINT64 default 4194304 # changing to
non-object size requires stripingv2 feature
For backwards compatibility, the default values preserve the existing
behavior of rbd_create(), but when you run qemu-img you could change
them to create format 2 images and potentially change the striping
as well. These options could be easily backported to cuttlefish
and bobtail.
What do you think?
The goal is to go from QCOW2 to RBD format 2 in one step.
[0]: http://tracker.ceph.com/issues/4897
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