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Problem with KVM version in Buildbot?


Hi Daniel,

We have a problem with a build in Buildbot:


+ qemu-img create -b /kvm/vms/vm-centos7_0-x86_64-build.qcow2 -f qcow2 /dev/shm/vm-tmp-2302.qcow2
qemu-img: '' uses a qcow2 feature which is not supported by this qemu version: QCOW version 3
qemu-img: Could not open '/kvm/vms/vm-centos7_0-x86_64-build.qcow2'
Failed to clone base image, aborting

This failure happens on the build host `terrier'.
I assume the problem is that the image was created on a machine with a newer
KVM? The host terrier seems to have a rather old one:

knielsen@terrier:~> kvm --version
QEMU emulator version 0.13.0 (qemu-kvm-devel), Copyright (c) 2003-2008 Fabrice Bellard

How do you think we should solve the problem? Should we upgrade terrier? Or
should we alternatively make sure to create new images in a
backward-compatible version (and re-create affected images) ?

Also, another issue related to VM images. Our old images were created with
rather small disk size. I noticed that some new images are created much
larger, for example:

$ qemu-img info /kvm/vms/vm-sid-amd64-build.qcow2
image: /kvm/vms/vm-sid-amd64-build.qcow2
file format: qcow2
virtual size: 40G (42949672960 bytes)
disk size: 3.1G
cluster_size: 65536
Format specific information:
    compat: 0.10

That is fine, I think, it is indeed annoying when we run out of space in a
build VM. However, the reason I noticed this is because host bb02 ran out of
space in /run/shm/ due to a 16 GB copy of this image.

So when we use larger VM images, we cannot use the optimisation to have the
temporary copies in ramdisk, lest we get disk full errors. If the virtual size
is larger to (ram disk size) / (max_builds), then we need to disable using
ramdisk for buildbot for that image.

I will try to fix the config to not use ramdisk for existing images that have
too large virtual size. But it is something to keep in mind for the future.

 - Kristian.

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