Live disk migration with Libvirt blockcopy

Nova uses libvirt blockcopy (python APIvirDomainBlockRebase)来做live snapshot.

Create the base image:

Qemu-img create-f Qcow2 Base 1G

Run PART-DISK/DEV/SDA MBR mkfs ext4/dev/sda1 mount/dev/sda1/ Touch/foo LS/ Exit

Create Another QCOW2 overlay snapshot ' Snap1 ' with backing file as ' base ':

Base.qcow2-o Backing_fmt=qcow2 Snap1.qcow2

Add a file to Snap1.qcow2:

Run PART-DISK/DEV/SDA MBR mkfs ext4/dev/sda1 mount/dev/sda1/ Touch/bar LS/ Exit

Create Another QCOW2 overlay snapshot ' snap2 ' with backing file as ' Snap1 ':

Qemu-img create-f qcow2-b snap1.qcow2-o backing_fmt=qcow2 snap2.qcow2

ADD another test file ' Baz ' into Snap2.qcow2 using guestfish (refer to previous examples above) to distinguish contents of BAS E, Snap1 and SNAP2.

Create a simple libvirt XML file as below, with source file pointing to Snap2.qcow2-which would be the active block Devic E (i.e. it tracks all new guest writes):

cat <<eof >/etc/libvirt/qemu/testvm.xml<domain type= ' KVM ' >  <name> testvm</name>  <memory unit= ' MiB ' >512</memory>     <vcpu>1</vcpu>  <os >    <type arch= ' x86_64 ' >hvm</type>  </os>  <devices>    <disk type= ' file ' Device= ' disk ' >      <driver name= ' qemu ' type= ' qcow2 '/>      <source file= '/export/vmimages/ Snap2.qcow2 '/>      <target dev= ' Vda ' bus= ' virtio '/>    </disk>     </devices></ domain>EOF

Define the Guest and start it:

Virsh define etc/libvirt/qemu/testvm.xml  Domain TESTVM defined from/etc/libvirt/qemu/virsh start TESTVMDomain TESTVM started

Perform Live Disk Migration
Undefine the running Libvirt guest to make it transient[*]:

virsh dumpxml--inactive testvm >/var/tmp/testvm.xml  Virsh undefine TESTVM

Check What's the current block device before performing live disk migration:

Virsh domblklist TESTVM Target     Source------------------------------------------------vda        /export/vmimages/snap2.qcow2

Optionally, display the backing chain of Snap2.qcow2:

qemu-img Info--backing-chain/export/vmimages/snap2.qcow2 [. . .] # Output removed for brevity

Initiate blockcopy (Live disk mirroring):

Virsh blockcopy--domain testvm vda/export/blockcopy-test/backups/copy.qcow2--wait--verbose--shallow--pivot 

Details of the above command: It creates copy.qcow2 file in the specified path; performs a --shallow blockcopy (i . E. The ' copy ' shares the backing chain) of the current block device ( vda ); –pivot'll pivot the live QEMU to the ' copy '.

Confirm that QEMU have pivoted to the ' copy ' by enumerating of the current block device on use:

Virsh domblklist TESTVM Target     Source------------------------------------------------vda        /export/vmimages/copy.qcow2 

Again, display the backing chain of ' copy ', it should is the resultant chain as noted in the Scenario section above).

qemu-img info--backing-chain/export/vmimages/copy.qcow2

Enumerate the contents of Copy.qcow2:

Run mount/dev/sda1/ LS/ quit

(You can notice above:all the content from Base.qcow2, Snap1.qcow2, and Snap2.qcow2 mirrored into copy.qcow2.)

Edit the Libvirt guest XML to use the Copy.qcow2, and define it:

Virsh Edit TESTVM <source file=‘/export/vmimages/snap2.qcow2‘/> <source file=‘/export/vmimages/copy.qcow2‘/> Virsh define/var/tmp/testvm.xml 

[*] Reason for the undefining and defining the guest again:as of writing this, QEMU have to support persistent dirty bitmap- This enables us to restart a QEMU process with disk mirroring intact. There is some in-progress patches upstream for a while. Until They is in Main line QEMU, the current approach (as illustrated above) Is:make a running Libvirt guest transient T Emporarily, perform live blockcopy , and make the guest persistent again. (Thanks to Eric Blake, one of the Libvirt project ' s principal developers, for this detail.)

http://kashyapc.com/2014/07/06/live-disk-migration-with-libvirt-blockcopy/

Live disk migration with Libvirt blockcopy