Re: Domain XML isn't dumping full backing chain

[Deepak Shetty <dpkshetty@xxxxxxxxx>]

Hmm interesting, so without adding backing_fmt, if i enable the below in /etc/libvirt/qemu.conf, will it work ?

# If allow_disk_format_probing is enabled, libvirt will probe disk
# images to attempt to identify their format, when not otherwise
# specified in the XML. This is disabled by default.
#
# WARNING: Enabling probing is a security hole in almost all
# deployments. It is strongly recommended that users update their
# guest XML <disk> elements to include  <driver type='XXXX'/>
# elements instead of enabling this option.
#
#allow_disk_format_probing = 1

On Wed, Mar 18, 2015 at 5:33 PM, Shanzhi Yu <shyu@xxxxxxxxxx> wrote:

---

From: "Deepak Shetty" <dpkshetty@xxxxxxxxx>
To: libvir-list@xxxxxxxxxx
Sent: Wednesday, March 18, 2015 7:19:05 PM
Subject: Domain XML isn't dumping full backing chain

Hi,

I am using libvirt version 1.2.9.2 on F21 and i am unable to get the complete backing chain info in the virsh dumpxml output. Details below :

My backing chain per qemu-img :

[stack@devstack-f21 test]$ qemu-img info --backing-chain snap4.qcow2
image: snap4.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: ./snap3.qcow2
Format specific information:
    compat: 1.1
    lazy refcounts: false

image: ./snap3.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: ./snap2.qcow2 (actual path: ././snap2.qcow2)
Format specific information:
    compat: 1.1
    lazy refcounts: false

image: ././snap2.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: ./snap1.qcow2 (actual path: ./././snap1.qcow2)
Format specific information:
    compat: 1.1
    lazy refcounts: false

image: ./././snap1.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
backing file: ./base.qcow2 (actual path: ././././base.qcow2)
Format specific information:
    compat: 1.1
    lazy refcounts: false

image: ././././base.qcow2
file format: qcow2
virtual size: 1.0G (1073741824 bytes)
disk size: 196K
cluster_size: 65536
Format specific information:
    compat: 1.1
    lazy refcounts: false

If you want prepare the backing chain yourself, you should  add "-o backing_fmt=$farmat" options,

like "qemu-img create -f qcow2 base.s1 -b base.qcow2 -o backing_fmt=qcow2"

I created 4 snapshots using commands (similar to the below, just dumping the last one here ):

[stack@devstack-f21 test]$ virsh snapshot-create-as test-domain on-test-snap4  --disk-only --reuse-external --diskspec hda,snapshot=external,file=/home/stack/test/snap4.qcow2
Domain snapshot on-test-snap4 created

[stack@devstack-f21 test]$ virsh domblklist test-domain
Target     Source
------------------------------------------------
hda        /home/stack/test/snap4.qcow2

virsh dumpxml test-domain|more
....

    <disk type='file' device='disk'>
      <driver name='qemu' type='qcow2' cache='none'/>
      <source file='/home/stack/test/snap4.qcow2'/>
      <backingStore type='file' index='1'>
        <format type='raw'/>

If you don't add "-o backing_fmt=qcow2", it will treat backing file format as raw format.

Since a raw format file doesn't have a backing file, so you only see snap4.qcow2 and

snap3.qcow2

        <source file='/home/stack/test/./snap3.qcow2'/>
        <backingStore/>
      </backingStore>
      <target dev='hda' bus='ide'/>
      <alias name='ide0-0-0'/>
      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
    </disk>

.....

I think thats happening beccause the backingStore format is coming as 'raw'

What am i missing ?

thanx,

deepak

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Regards
shyu

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