[SOLVED] CentOS 6 copied disk not converting with qemu-img

skagnola · 04-27-2018, 09:45 PM

Hello!

Running into an odd issue. I copied a couple VM disks using dd. That went fine. Output size and all is about where it should be.

I then went to info one of the images, and it came back as expected...

Code:

qemu-img info disk_rt.img
image: disk_rt.img
file format: qcow2
virtual size: 10G (10737418240 bytes)
disk size: 12G
cluster_size: 65536
Format specific information:
    compat: 0.10
    refcount bits: 16

Next was converting it, but this is where the issue lies. Upon initiating the conversion, the task completes instantly and is only 5MB. No errors. This disk was added to the root lv some time in the past, expanding its space.

Code:

qemu-img convert -p -f qcow2 -O vmdk disk_rt.img disk_rt_vda

Code:

5.0M Apr 28 02:08 disk_rt_vda

The OS disk went fine with converting. It was also a qcow2, of course.

Code:

qemu-img info disk_os.img
image: disk_os.img
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 25G
cluster_size: 65536
Format specific information:
    compat: 0.10
    refcount bits: 16

Code:

qemu-img convert -p -f qcow2 -O vmdk disk_os.img disk_os_vda

Code:

18G Apr 28 02:07 disk_os_vda

Not really sure if because of this added in disk being part of a partitioned OS disk, a separate disk set up as an lvm to supplement, is causing this?

For some reason qemu-img is not seeing much for it to convert.

rknichols · 04-28-2018, 09:22 AM

Perhaps very little of that new disk was actually used. The conversion only processes clusters that are actually mapped, i.e., clusters that were written to at least once since the volume was created.

skagnola · 04-28-2018, 10:01 AM

Quote:

Originally Posted by rknichols

Perhaps very little of that new disk was actually used. The conversion only processes clusters that are actually mapped, i.e., clusters that were written to at least once since the volume was created.

Huh, that's interesting. Did not know that. I guess I will continue with the process and see what happens.

rknichols · 04-28-2018, 11:22 AM

FWIW, I just tried creating a 10GB qcow2 file with option "preallocation=full" so that it took up the full 10GB on disk, did nothing but create a partition formatted to ext4. "qemu-img map --output=json" showed every cluster with '"zero": false, "data": true"'. Running "qemu-img convert" to convert it to vmdk yielded a tiny file. Converting that tiny vmdk file back to qcow2 almost instantly yielded a tiny qcow2 file that nonetheless contained the partitioning and filesystem that I had created.

skagnola · 04-28-2018, 05:25 PM

rknichols thanks for the info. I went ahead with the disk imports/clones and the VM came up fine!

Due to my lack of experience with qemu-img, I figured the output file size noted in the OS would be close to the actual full disk size following the conversion. Definitely good info to know only mapped clusters are worked.

;-)