External(and Live)snapshots with libvirt

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Raw image is a blob of data exposed directly in VM as block device，it can't snapshot.qem-i mg is able to imposed data from raw image into qcow 2 image，and save it as snapshot
Previouslogy，I posted about s snapshots here、which breieflydiscussed different types of snapshots.In this post，let’s explore how external sssshotswork. Just to quickreshsh、emamash thethethethethemamaststststststststststststininininininininine thethethethethethethethethethethethethethemamamamamamamash ssssssssssssssssssssssssssssssssssssssssence/delta（aka，the snapshot image）is stored in a new QCOW 2 file.Onece the snapshot is taken，the original disk mage will be in a‘read-only’state，which can be used as backing file for other ests.
It’s wortth mentioning here that:
  • The original disk image can be either in RAW format or QCOW 2 format.When a snapsphot is tane，the difference'will be stored in a different QCOW 2 file
  • The virtual machine has to be running、ライブ.Also with Live snapshots、no gust dontime is experienced when a snapshot is taken.
  • At this moment，external(Live)snapshots work for'disk-only'snapshots(and not VM state).Work for both disk and VM state(and also，reverting to external disky.inpshot state)insp.infont.infont.infont.infont.infont.infont.infont.infont.infont.infont.infont.ings.infont.in
    Before we go aead、here’s some version info、I’m testing on Fedora-17（host）、and the gest（named‘daisy’）is running Fedora-18（Test Compse）：
    
[root@moon ~]# rpm -q libvirt qemu-kvm ; uname -r
libvirt-0.10.1-3.fc17.x86_64
qemu-kvm-1.2-0.2.20120806git3e430569.fc17.x86_64
3.5.2-3.fc17.x86_64
[root@moon ~]#
    External disk-snapshots(live)using QCOW 2 as original mage:Let’s see an illustration of external(live)disk-only snapshots.First，let’s ensure the gust is running:
    
[root@moon qemu]# virsh list
 Id    Name                           State
----------------------------------------------------
 3     daisy                          running


[root@moon qemu]#
    The n、list all the block devices assicated with the gusest:
    
[root@moon ~]# virsh domblklist daisy --details
Type       Device     Target     Source
------------------------------------------------
file       disk       vda        /export/vmimgs/daisy.qcow2

[root@moon ~]#
    Next、let’s create a snapshot（disk-only）of the gust this way、while the gust is running：
    
[root@moon ~]# virsh snapshot-create-as daisy snap1-daisy "snap1 description" \
 --diskspec vda,file=/export/vmimgs/snap1-daisy.qcow2 --disk-only --atomic
    Some details of the flags used:-Passing a'–diskspec’parameter adds the'disk'elemens to the Snapshot XML file-'–disk-only parameter，tas the snapshot of onlythe diskes-the the the the the the magenese'jec
    Let’s check the information aout the just taken snapshot by running qem-i mg：
    
[root@moon ~]# qemu-img info /export/vmimgs/snap1-daisy.qcow2 
image: /export/vmimgs/snap1-daisy.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 2.5M
cluster_size: 65536
backing file: /export/vmimgs/daisy.qcow2
[root@moon qemu]#
    Appt from the above，I created 2 more snapshots(just the same sysntax as above)for illustration purpose.Now，the snapshot-tree look s like this:
    
[root@moon ~]# virsh snapshot-list daisy --tree

snap1-daisy
  |
  +- snap2-daisy
      |
      +- snap3-daisy
        

[root@moon ~]#
    For the above example image file chain[base]<-snap 1<-snap 3]，it has to be read as–snap 3 has snap 2 as its backing file，snap 2 has snap 1 asits backing file，and snap 1 file
    
#--------------------------------------------#
[root@moon ~]# qemu-img info /export/vmimgs/snap3-daisy.qcow2
image: /export/vmimgs/snap3-daisy.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 129M
cluster_size: 65536
backing file: /export/vmimgs/snap2-daisy.qcow2
#--------------------------------------------#
[root@moon ~]# qemu-img info /export/vmimgs/snap2-daisy.qcow2
image: /export/vmimgs/snap2-daisy.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 3.6M
cluster_size: 65536
backing file: /export/vmimgs/snap1-daisy.qcow2
#--------------------------------------------#
[root@moon ~]# qemu-img info /export/vmimgs/snap1-daisy.qcow2
image: /export/vmimgs/snap1-daisy.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 2.5M
cluster_size: 65536
backing file: /export/vmimgs/daisy.qcow2
[root@moon ~]#
#--------------------------------------------#
    Now、if we do not need snap 2 any more、and want to pull the data from snap 1 into snap 3、making snap 1 a s snap 3’s backing file、we can do a virsh blockpuration as below：
    
#--------------------------------------------#
[root@moon ~]# virsh blockpull --domain daisy  --path /export/vmimgs/snap3-daisy.qcow2 \
 --base /export/vmimgs/snap1-daisy.qcow2 --wait --verbose
Block Pull: [100 %]
Pull complete
#--------------------------------------------#
    Where、–path=path to the snapshot file、and–base=path to a backing file from whihich the datato to be putled.So from abooeexample、it’s e e e e e e e e e evident that t aaaaaat the pplling the dadadadadadadadadadadadataffffrom sfffrom snapsnapppnapppnapppppppppnananapppppppppppppppnanatitititishshshshshshshshshshshshshshshshshshshshshnning qem-i mg again.Thing to note here
    
[root@moon ~]# qemu-img info /export/vmimgs/snap3-daisy.qcow2 
image: /export/vmimgs/snap3-daisy.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 145M
cluster_size: 65536
backing file: /export/vmimgs/snap1-daisy.qcow2
[root@moon ~]#
    A couple of things to note here、after discussion with Erric Blake（thank you）：
    -If we do a listing of the snapshot tree again（now that‘snap 2-daisy.qcow 2’backing file is no more in use）、
    
[root@moon ~]# virsh snapshot-list daisy --tree
snap1-daisy
  |
  +- snap2-daisy
      |
      +- snap3-daisy
[root@moon ~]#
    might wonder、why is s snap 3 still poininininingto snap 2？Thing to note hee is、the abobove isthe snapshot chain、which isindededededededededededededefm discaaatdisssssbacking file chain.So、the、the、the'virsrsrss the's the's the's the's the's the's the's s s s s s s s s s s s thethethe the the the the the the the the the thethethethe isisisisisisisisisisisininininininininer creating the snapshot）So，from the above snapshot tree，if we we to revert to snap 1 or snap 2(when revert-to-disk-snapshots is available)、it'd still be possible to do that，meaning:
    It’s possible to go from this state：base<-snap 123(data from snap 1,snap 2 pulled into snap 3)
    we can still revert to:
    base<-snap 1(thus undong the changes in snap 2&snap 3)
    External disk-snapshots(ライブ)using RAW as original mage:With external disk-snapshots，the backing file can be RAW as well
    A quick illustration below.The command are self-explannary.It can be noted the change(from RAW to QCOW 2)in the block disk disk the Grest，before&after Taning the disk-snapshot
    
#-------------------------------------------------#
[root@moon ~]# virsh list | grep f17btrfs2
 7     f17btrfs2                      running
[root@moon ~]#
#-------------------------------------------------#
[root@moon ~]# qemu-img info /export/vmimgs/f17btrfs2.img                                                                                                                                                      
image: /export/vmimgs/f17btrfs2.img
file format: raw
virtual size: 20G (21474836480 bytes)
disk size: 1.5G
[root@moon ~]# 
#-------------------------------------------------#
[root@moon qemu]# virsh domblklist f17btrfs2 --details
Type       Device     Target     Source
------------------------------------------------
file       disk       hda        /export/vmimgs/f17btrfs2.img

[root@moon qemu]# 
#-------------------------------------------------#
[root@moon qemu]# virsh snapshot-create-as f17btrfs2 snap1-f17btrfs2 "snap1-f17btrfs2-description" \
--diskspec hda,file=/export/vmimgs/snap1-f17btrfs2.qcow2 --disk-only --atomic
Domain snapshot snap1-f17btrfs2 created
[root@moon qemu]# 
#-------------------------------------------------#
[root@moon qemu]# qemu-img info /export/vmimgs/snap1-f17btrfs2.qcow2 
image: /export/vmimgs/snap1-f17btrfs2.qcow2
file format: qcow2
virtual size: 20G (21474836480 bytes)
disk size: 196K
cluster_size: 65536
backing file: /export/vmimgs/f17btrfs2.img
[root@moon qemu]# 
#-------------------------------------------------#
[root@moon qemu]# virsh domblklist f17btrfs2 --details
Type       Device     Target     Source
------------------------------------------------
file       disk       hda        /export/vmimgs/snap1-f17btrfs2.qcow2
[root@moon qemu]# 
#-------------------------------------------------#
    Also note:All snapshot XML files，where libvirt trocs the metadata of snapshots are located under/var/libvirt/qem/snapshots/USgust name
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