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[Qemu-devel] Request for clarification on qemu-img convert behavior zero

From: De Backer, Fred (Nokia - BE/Antwerp)
Subject: [Qemu-devel] Request for clarification on qemu-img convert behavior zeroing target host_device
Date: Thu, 13 Dec 2018 13:12:12 +0000


We're using Openstack Ironic to deploy baremetal servers. During the deployment 
process an agent (ironic-python-agent) running on Fedora linux uses qemu-img to 
write a qcow2 file to a blockdevice.

Recently we saw a change in behavior of qemu-img. Previously we were using 
Fedora 27 containing a fedora packaged version of qemu-img v2.10.2 
(qemu-img-2.10.2-1.fc27.x86_64.rpm); now we use Fedora 29 containing a fedora 
packaged version of qemu-img v3.0.0 (qemu-img-3.0.0-2.fc29.x86_64.rpm).

The command that is run by the ironic-python-agent (the same in both FC27 and 
FC29) is: qemu-img -t directsync -O host_device /tmp/image.qcow2 /dev/sda

We observe that in Fedora 29 the qemu-img, before imaging the disk, it fully 
zeroes it. Taking into account the disk size, the whole process now takes 35 
minutes instead of 50 seconds. This causes the ironic-python-agent operation to 
time-out. The Fedora 27 qemu-img doesn't do that.

Scanning through the qemu-img source code, we found that adding -S 0 to the 
command on Fedora 29 qemu-img restores the behavior as observed in Fedora 27 

Looking through the changelogs of qemu I couldn't find this behavior change 

Now the questions:
* Is this the expected/required behavior that qemu-img first zeroes the 
complete target disk before writing the image. In other words: is this a 
qemu-img bug?
* Is applying the -S 0 parameter a safe/sound/sensible thing to do to revert to 
the old behavior. In other words: can I write a bug against the 
ironic-python-agent to start using this parameter?
* If the behavior is expected: is there some pointer to 
documentation/changelogs I can read about this?


Fred De Backer
SME Video Integration Engineer,
IP/Optical Networks, Nokia

Nokia Bell NV I Copernicuslaan 50, 2018 Antwerpen I BTW BE 0404 621 642 RPR 
Antwerpen I BNP Paribas Fortis 220-0002334-42 I IBAN BE 77 2200 0023 3442 I BIC 

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