[libvirt] Domain XML isn't dumping full backing chain

Shanzhi Yu shyu at redhat.com
Wed Mar 18 12:03:06 UTC 2015


----- Original Message -----

| From: "Deepak Shetty" <dpkshetty at gmail.com>
| To: libvir-list at redhat.com
| Sent: Wednesday, March 18, 2015 7:19:05 PM
| Subject: [libvirt] 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 at 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 at 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 at 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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