Questions[qcow2](server)

I'm trying to get the virtual size of a qcow2 container down so that it can fit in a openstack flavor with 10GB disk.

root@node-10:~# qemu-img info zztop.qcow 
image: zztop.qcow
file format: qcow2
virtual size: 80G (85899345920 bytes)
disk size: 2.6G
cluster_size: 65536
Format specific information:
compat: 1.1
lazy refcounts: false

I follow the guide here and fill the end of the disk with zeros then run qemu-img convert -O qcow2. The resulting qcow2 however retains the same disk size and virtual size.

How do I lower the virtual size of a qcow2 so that it can fit in a openstack flavor smaller than 80GB?

I'm having some serious disk performance problems while setting up a KVM guest. Using a simple dd test, the partition on the host that the qcow2 images reside on (a mirrored RAID array) writes at over 120MB/s, while my guest gets writes ranging from 0.5 to 3MB/s.

• The guest is configured with a couple of CPUs and 4G of RAM and isn't currently running anything else; it's a completely minimal install at the moment.
• Performance is tested using time dd if=/dev/zero of=/tmp/test oflag=direct bs=64k count=16000.
• The guest is configured to use virtio, but this doesn't appear to make a difference to the performance.
• The host partitions are 4kb aligned (and performance is fine on the host, anyway).
• Using writeback caching on the disks increases the reported performance massively, but I'd prefer not to use it; even without it performance should be far better than this.
• Host and guest are both running Ubuntu 12.04 LTS, which comes with qemu-kvm 1.0+noroms-0ubuntu13 and libvirt 0.9.8-2ubuntu17.1.
• Host has the deadline IO scheduler enabled and the guest has noop.

There seem to be plenty of guides out there tweaking kvm performance, and I'll get there eventually, but it seems like I should be getting vastly better performance than this at this point in time so it seems like something is already very wrong.

Update 1

And suddenly when I go back and test now, it's 26.6 MB/s; this is more like what I expected w/qcrow2. I'll leave the question up in case anyone has any ideas as to what might have been the problem (and in case it mysteriously returns again).

Update 2

I stopped worrying about qcow2 performance and just cut over to LVM on top of RAID1 with raw images, still using virtio but setting cache='none' and io='native' on the disk drive. Write performance is now appx. 135MB/s using the same basic test as above, so there doesn't seem to be much point in figuring out what the problem was when it can be so easily worked around entirely.

I want to recreate a dynamically allocated qcow2 image in order to shrink it. Is it sufficient that all unnecessary files have been deleted, or do I also need to fill the space formerly occupied by those files with zeros? In other words, is qemu-img filesystem-aware?

we are using kvm/qemu with qcow2-images for our virtual machines.

qcow2 has this nice feature where the image file only allocates the actually needed space by the virtual-machine. but how do i shrink back the image file, if the virtual machine's allocated space gets smaller?

example:

1.) i create a new image with qcow2 format, size 100GB

2.) i use this image to install ubuntu. installation needs about 10 gb, the image-file grows up to about 10GB. nothing unexpected so far.

3.) i fill up the image with about 40 GB of additional data. the image-file grows up to 50GB. i am ok with that :-)

4.) this is where it gets strange: i delete all of the 40GB data on the image, but the image-size still eats up 50GB.

question: how do i free up that 40GB of data and shrink the image to the only needed 10 GB?

thanks in advance, berni

I'm running some Xen-servers and started migrating to KVM.

Currently my guests are either running on raw-images or LVMs.

I found libvirt providing some very nice snapshot features (virsh snapshot-create, ...) so I decided to use qcow2 instead of raw/lvm.

And here is my question: libvirt creates the same sort of snapshots on the qcow2 image as if I use qemu-img - is it possible to mount them ? I read something about qemu-nbd and the possibility of mounting qcow but I could not find a word about snapshots.