問題描述：
在ovirt中，建立快照後刪除該快照失敗，只有重啟虛擬機器後該快照才能刪除

日誌中均出現如下報錯：

2018-02-19 01:51:45,792+0800 ERROR (jsonrpc/7) [virt.vm] (vmId=’a5935784-6d8a-4a4e-ab18-bb8c9b8cce9b’) Live merge failed (job: 7b217072-2149-4439-ab28-4a8bc52ada3d) (vm:4918)
Traceback (most recent call last):
File “/usr/share/vdsm/virt/vm.py”, line 4914, in merge
flags)
File “/usr/lib/python2.7/dist-packages/vdsm/virt/virdomain.py”, line 69, in f
ret = attr(*args, **kwargs)
File “/usr/lib/python2.7/dist-packages/vdsm/libvirtconnection.py”, line 123, in wrapper
ret = f(*args, **kwargs)
File “/usr/lib/python2.7/dist-packages/vdsm/utils.py”, line 926, in wrapper
return func(inst, *args, **kwargs)
File “/usr/lib/python2.7/dist-packages/libvirt.py”, line 674, in blockCommit
if ret == -1: raise libvirtError (‘virDomainBlockCommit() failed’, dom=self)
libvirtError: Requested operation is not valid: can’t keep relative backing relationship

問題使用libvirt virsh復現步驟：

$ cat build.sh 
#!/bin/bash

sudo qemu-img create -f qcow2 base.qcow2 10G
#sudo qemu-img create -f qcow2 sn1.qcow2 -o backing_file=base.qcow2 10G

sudo virsh define /opt/libvirt/vm1.xml 
sudo virsh start vm1

sudo virsh domblklist vm1

sudo virsh snapshot-create-as --domain vm1 snap1 \
    --diskspec vda,file=/opt/libvirt/sn1.qcow2 \
    --disk-only --atomic --no-metadata

sudo
 
 virsh snapshot-create-as --domain vm1 snap2 \
    --diskspec vda,file=/opt/libvirt/sn2.qcow2 \
    --disk-only --atomic --no-metadata

sudo virsh snapshot-create-as --domain vm1 snap3 \
    --diskspec vda,file=/opt/libvirt/sn3.qcow2 \
    --disk-only --atomic --no-metadata

sudo virsh domblklist vm1

sudo
 
 qemu-img info --backing-chain \
     /opt/libvirt/sn3.qcow2

sudo virsh domblklist vm1

#sudo virsh blockcommit vm1 vda --verbose --pivot --active

echo "-------merge"

sudo virsh blockcommit --domain vm1 vda --base /opt/libvirt/sn1.qcow2  \
     --top /opt/libvirt/sn2.qcow2 --keep-relative --wait --verbose

sudo virsh domblklist vm1

sudo qemu-img info --backing-chain /opt/libvirt/vm1.xml


$ cat unbuild.sh 
#!/bin/bash

sudo virsh destroy vm1
sudo virsh undefine vm1
sudo rm ./*.qcow2


$ cat vm1.xml 
<domain type='kvm'>  //如果是Xen，則type=‘xen’
    <name>vm1</name> //虛擬機器名稱，同一物理機唯一
    <uuid>fd3535db-2558-43e9-b067-314f48211343</uuid>  //同一物理機唯一，可用uuidgen生成
    <memory>524288</memory>
    <currentMemory>524288</currentMemory>  //memory這兩個值最好設成一樣
    <vcpu>2</vcpu>            //虛擬機器可使用的cpu個數，檢視物理機可用CPU個數：cat /proc/cpuinfo |grep processor | wc -l 
    <os>
        <type arch='x86_64' machine='pc-i440fx-vivid'>hvm</type> //arch指出系統架構型別，machine 則是機器型別，檢視機器型別：qemu-system-x86_64 -M ?
        <boot dev='hd'/>  //啟動介質，第一次需要裝系統可以選擇cdrom光碟啟動
        <bootmenu enable='yes'/>  //表示啟動按F12進入啟動選單
    </os>
    <features>
        <acpi/>  //Advanced Configuration and Power Interface,高階配置與電源介面
        <apic/>  //Advanced Programmable Interrupt Controller,高階可程式設計中斷控制器
        <pae/>   //Physical Address Extension,實體地址擴充套件
    </features>
    <clock offset='localtime'/>  //虛擬機器時鐘設定，這裡表示本地本機時間
    <on_poweroff>destroy</on_poweroff>  //突發事件動作
    <on_reboot>restart</on_reboot>
    <on_crash>restart</on_crash>
    <devices>   //裝置配置
        <emulator>/usr/bin/kvm</emulator> //如果是Xen則是/usr/lib/xen/binqemu-dm
        <disk type='file' device='disk'> //硬碟
            <driver name='qemu' type='raw'/>
            <source file='/opt/libvirt/base.qcow2'/>
            <target dev='vda' bus='virtio'/>
            <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/> //域、匯流排、槽、功能號，slot值同一虛擬機器上唯一
        </disk>
        <disk type='file' device='cdrom'>//光碟
            <driver name='qemu' type='raw'/>
            <source file='/home/pencc/圖片/Linx-Node-201711-amd64.iso'/>
            <target dev='hdc' bus='ide'/>
            <readonly/>
        </disk>
        <graphics type='vnc' port='5900' autoport='yes' listen='0.0.0.0' keymap='en-us'>  //配置vnc，windows下可以使用vncviewer登入，獲取vnc埠號：virsh vncdisplay vm0
            <listen type='address' address='0.0.0.0'/>
        </graphics>
  </devices>
</domain>

通過gdb除錯排查，發現問題在於建立快照後該快照結構體中的relPath為null，重啟虛擬機器後能得到該relPath，說明只有重啟後重構快照鏈時才會賦值，而真實的relPath儲存在dd->disk->src->relPath中，故在建立快照鏈流程中的qemuDomainSnapshotDiskDataCollect中新增VIR_STEAL_PTR即可。

修改後實測多次均ok。